Justin winning the first fight was a "holy fuck". Fiziev was putting it on him and was too fast for Justin. Justin showed what a formidable, high level fighter he is by surviving, getting the data and putting it on Fiziev later in the fight. It's probably Justin's most impressive win, not in terms of name value but showing how great a standup fighter he is, across a whole fight. Which is to say, beating Fiziev was a tall task.
I would favour Gaethje to be able to do it again, unless the Max fight and KO really affected him (this is why the KO was a mix of holy fuck and "damn that sucks", for a guy at his age/advanced career stage where a KO can really have an affect on their chin). I know Fiziev said the eye poke lowered his performance though. If that's true and it wasn't simply Fiziev fading, then Justin is in for a harder fight.
Justin's probably improved a little (eg look at the Poirier head kick), but idk if those improvements are things that would trouble Fiziev and make the fight look different.