There's a lot to unpack here.
1. Arkle's race, the Gallaher Gold Cup (correct spelling) was the shorter race by listed distance according to the Racing Post.
2. However it was run from the same starting point as the Gainsborough (Agfa Diamond from 91-07) as far as I can tell. The time to the first fence is very similar for both races (Tanlic must be referring to a video that omits the first circuit) as can be seen in the linked videos. I have done some sleuthing and it seems the RP have added the exact distance to Arkle's race in line with the current listed distance for the 3m chase course at Sandown. The distance is listed as 3m118y on the live broadcast (later 3m110y or 3m1/2f in the 90's) for Desert Orchid but it's the same starting point.
The distance is also listed as 3m118y for the 1965 Gainsborough Chase: https://www.worthpoint.com/worthopedia/sandown-12th-february-1965-mill-house-1776002927
3. There are definitely jump cuts in the 1965 youtube video. It seems that the BBC had O'Sullevan commentate over recorded footage (Sandown wasn't a BBC track at that time, so wasn't live on the channel). They must only have had raw footage from several cameras and attempted to splice them together after reports of Arkle's feat. Whether they cut sections out in error or for brevity it doesn't particularly matter; I doubt people had stop watches going at home while they were watching. It was thrashing Mill House while giving a lot of weight that made the performance so impressive.
Any sectionals can only be done when Arkle's race has a continuous camera shot; so not much can really be achieved, but the starting point can be established.
https://youtu.be/tamkrqCAegs?t=45
This can be compared to Desert Orchid in 1987 https://youtu.be/jaeJKmJXI-Y?t=30 and the 1967 Whitbread https://youtu.be/FrDRq0QZfvI?t=82 (note the wooden tower).
Small aside 1: One fence is omitted in the 1965 race from the footage (the second).
Small aside 2: All the railway fences are shown, you can pause at 1:39 and 4:40 to see the first of them with a parked car visible in the background. It's unclear if only the last two are classified as railway fences by O'Sullevan (as may have been the standard at the time) as he actually misidentifies them on the second circuit (as-live commentary over recorded footage with parts missing obviously isn't easy).
Small aside 3: The fences, although in the same place, were noticeably different in structure between the 60's and 80's (a lot easier to jump in Arkle's day).
As far as breaking the course record by 17 seconds I haven't been able to find any context or evidence other than it was set by Mill House in 1964 (I can't even find the actual time or the current course record). The Gallaher Gold Cup was a very new race so it is plausible that on fast early November ground, aided by an end to end gallop and with few 3m chases previously run outside of the depth of winter at Sandown that this was more or less true. Or perhaps that it was simply 17 seconds faster than Mill House's win in the 1964 edition.
Someone with more knowledge and our resources may be able to assist with figuring it out exactly.
1. Arkle's race, the Gallaher Gold Cup (correct spelling) was the shorter race by listed distance according to the Racing Post.
2. However it was run from the same starting point as the Gainsborough (Agfa Diamond from 91-07) as far as I can tell. The time to the first fence is very similar for both races (Tanlic must be referring to a video that omits the first circuit) as can be seen in the linked videos. I have done some sleuthing and it seems the RP have added the exact distance to Arkle's race in line with the current listed distance for the 3m chase course at Sandown. The distance is listed as 3m118y on the live broadcast (later 3m110y or 3m1/2f in the 90's) for Desert Orchid but it's the same starting point.
The distance is also listed as 3m118y for the 1965 Gainsborough Chase: https://www.worthpoint.com/worthopedia/sandown-12th-february-1965-mill-house-1776002927
3. There are definitely jump cuts in the 1965 youtube video. It seems that the BBC had O'Sullevan commentate over recorded footage (Sandown wasn't a BBC track at that time, so wasn't live on the channel). They must only have had raw footage from several cameras and attempted to splice them together after reports of Arkle's feat. Whether they cut sections out in error or for brevity it doesn't particularly matter; I doubt people had stop watches going at home while they were watching. It was thrashing Mill House while giving a lot of weight that made the performance so impressive.
Any sectionals can only be done when Arkle's race has a continuous camera shot; so not much can really be achieved, but the starting point can be established.
https://youtu.be/tamkrqCAegs?t=45
This can be compared to Desert Orchid in 1987 https://youtu.be/jaeJKmJXI-Y?t=30 and the 1967 Whitbread https://youtu.be/FrDRq0QZfvI?t=82 (note the wooden tower).
Small aside 1: One fence is omitted in the 1965 race from the footage (the second).
Small aside 2: All the railway fences are shown, you can pause at 1:39 and 4:40 to see the first of them with a parked car visible in the background. It's unclear if only the last two are classified as railway fences by O'Sullevan (as may have been the standard at the time) as he actually misidentifies them on the second circuit (as-live commentary over recorded footage with parts missing obviously isn't easy).
Small aside 3: The fences, although in the same place, were noticeably different in structure between the 60's and 80's (a lot easier to jump in Arkle's day).
As far as breaking the course record by 17 seconds I haven't been able to find any context or evidence other than it was set by Mill House in 1964 (I can't even find the actual time or the current course record). The Gallaher Gold Cup was a very new race so it is plausible that on fast early November ground, aided by an end to end gallop and with few 3m chases previously run outside of the depth of winter at Sandown that this was more or less true. Or perhaps that it was simply 17 seconds faster than Mill House's win in the 1964 edition.
Someone with more knowledge and our resources may be able to assist with figuring it out exactly.