They knew from the Guineas, in which he overtook Kingman in the final furlong, that he had plenty of pace. They clearly thought if they had a lead on him off a slower pace and got that kick going it would be unlikely, based on the Newmarket run, that Kingman wouldn't be able to give him a start, first run and a beating.
Hughes has already said he couldn't believe how fast he was travelling at Ascot - suggesting he got the tactics spot on - only for Kingman to fly past him. They just didn't realise Kingman didn't run to form in the Guineas and the Irish form was inconclusive.
They know now [how good Kingman is], which is maybe another factor in the decision to try him at 10f. He has very good mile speed, can quicken and is bred for the longer trip, so the Eclipse is a perfectly reasonable next step. I mentioned on here before the Derby that I hoped SteveM was badgering connections to run NOT at Epsom so I don't see this race being an issue.