So does chess dot com use a server-based, much stronger engine when doing game review? I suppose that’s why it doesn’t work offline? The reason I’m asking is that when playing against the best engine it’s not making all the best moves and also makes some mistakes etc.

Be specific.     

leonasskau.co.uk //
Hosting /strategy, /geopolitics d33m:mun

Yeah I suppose they could in theory let people play against the Game Review engine in which case it would by definition get 100% best moves always. But like you said it’s not really needed since the best locally running engine will beat any human player anyway so why waste the server resources?

Wouldn’t be surprised if the best engine a user can play against is not The Best Possible Chess Player Ever

Quite possible they just thought there isn’t a use case to let users lose to a flawless engine