I mean, it was only part of one round so obviously grain of salt, but look at Tony after being mounted by Lee. Scrambling, bucking, finally getting guard back as round 1 ended. Lee maybe isn't Khabib, but when fresh he's a beast on the mat (look what he did to Chiesa). Tony popped up at the end of that round like he'd just been chilling on his couch. Literally not even breathing hard at all, just fine.
And I realize it's sort of projection to a degree, but if you look at RDA, he took less damage in round 3 vs Khabib than in the first 2 round, and was able to get back to his feet. Khabib slowed a bit in round 3 there. I don't think it's irrational to think that Tony has better cardio than RDA, regardless of where the fight is taking place.
I think the bigger issue for Tony is his tendency to take silly chances and get stuck in bad spots, always just trusting his durability and creativity to get him out of them. He needs to be pretty damn careful with a grappler the level of Khabib in doing that stuff.