While I think Thomas will be dangerous, I don't get why you keep bringing up 2018. Firstly, Thomas has never achieved that level again after that. Secondly, Roglič is a different beast now than he was in 2018. He wasn't even supposed to ride for GC back then.
This is a common theme now, i.e. Ineos winning races where the aforementioned Slovenian terminators aren't racing & suddenly some people label them Tour favorites.
Thomas deserves some serious respect for reaching this kind of level again (certainly after the crash in the Giro last year would have demoralized some lesser riders), but it does him no favors whatsoever to say he's suddenly better than Primoz Roglic & back in the hunt for yellow in Paris.
Roglic & Pogacar in 2021 are like peak Contador versus peak Schleck, or peak Pantani versus peak Ullrich, or peak Lance versus peak whomever (ignore the clinic implications, I'm talking actual performance level with regards to the competition).
They'll fly away on the summit finishes like they did last year (like when Bernal started whining about his best numbers not being enough). Thomas is a level below them.