If he wanted to dive could have just done it in round 3 after he got hurt.
The performance of a career without a doubt. For the first time for me he looked like (and is) a championship fighter. Jabbed right away, used his wrestling (which he famously hates doing) to get to the clinch (which he likes), had to weather a really cool bit of offence in round 2 from Ilia that I thought was close to having it stopped (and apparently after the fight in the presser, said his guys were all kinds of fucked up).
Ilia on the other hand looked like he believed this was a squash match and flung himself at Gaethje trying to kill him in 2 rounds. Gassed and (if translations are correct) had to be saved by his brother in round 4. You look at some of his other fights where he really respects the opponent, his defence is a lot more on point.
That body shot flurry Topuria threw in round 2 was the finishing move, the man put everything in those shots, and Justin managed to keep the lights on, even if barely.
A bunch of people said that Ilia fumbled when he went to grapple after those shots when Justin was hurt, but I think that was Ilia going for a body lock or choke atempt to see if he could get some gas back, because he was dead after that, he didn't have the power to finish Gaethje.
And that is the thing, Justin is a power puncher and this is the first time Ilia got to see what being hit in the lightweights is really like. Charles also hits hard, but his fight with Topuria ended was too fast. After a couple of stiff jabs from Gaethje, Topuria looked surprised, and this is not good.
Also, I think Justin cracked Topuria's ribs with a knee on R4, that is why the call was made.
But all the credit goes to Gaethje, how he survived those bodyshots is beyond me, he should've have folded right then and there.
It was indeed a Cinderella night for the man, stuff that can only happen in combat sports.