Just catching up with this - there were some pretty big names involved. And looking at the Zwiftpower results some pretty big w/kg too - very impressive! Ian Bibby who won it averaged 5.2w/kg for the 50 minutes and there were a few others even higher. Freddy Ovett (son of Steve Ovett) averaged 5.8w/kg and Connor Swift averaged 418 watts overall ( a measly 5.5w/kg!). Puts our measly efforts into context!
Its the W/Kg and HR below peak. The power numbers were 350-450 mostly, but they are able to do that some 20bpm below peak.