They're correct with the Slayer album at number 1. It's dated a bit now but when it came out I'd never heard anything as intense and fast.
The Mastodon and Slipknot albums are not heavy at all.
Sleep and Electric Wizard are good shouts. Proper doom merchants.
Because of when it came out the first Black Sabbath album should be number 2. For 1970 it still sounds heavy to this day.
No proper Death Metal album, no Pleasure to Kill by Kreator, no To Mega Therion by Celtic Frost and no Black Metal albums are notable, poor omissions.
I can understand most people not "getting" Metal, but everyone should hear and try and digest Reign In Blood - it was a stunning release.