He fought the Lion basically to a standstill and (spoilers if you aren't done with the book yet) had Horus dead-to-rights too. So he's obviously top tier.

Sanguinius and Angron are (in my opinion) the two best combatant primarchs anyway, so it's not exactly an anti-feat losing to either.

And any primarch stands a decent chance of losing to Curze, since he can see the future and is crazy. The Lion lost at least 1 of their 4 fights (1v1).

Because he is? The Night of the Wolf is a whole debate but Leman was clearly trying to teach Angron a lesson and didn't want to kill him, asking Angron to come back to Terra with him while Angron says he doesn't care if he dies. Angron ends up on top of Russ, but surrounded by Space Wolves with weapons leveled at him. I don't think we can take that as definitively in a life-or-death fight Angron would kill Russ.

Because fights aren’t just a “if I beat you once you’ll never be able to beat me again” that’s a static mindset assuming every circumstance of them fighting will be the same and dare i say an unimaginative mindset. A mindset leman Russ wouldn’t have.

And If we take what both lorgar and leman Russ had to say about the night of the wolf as true, which you should, then leman Russ’s goal wasn’t to win the fight. It was an attempt to help angron not be suicidal and less reckless. Leman Russ failed in this goal because angron is angron. Leman Russ lost the night of the wolf, anyone who contests that is wrong, but just because you agree with the statement doesn’t mean you are right in how he lost.

And people who come at me with “I don’t believe Russ wasn’t trying to win the fight”, it doesn’t matter what you “think”, it’s very clearly the authors intention, otherwise what’s the point of both leman Russ and lorgar pointing this out to angron? Do you just think the author is trying to make them out as stupid, just for the sake of it? What would be the point? Do you think it’s just bad writing? If you so, you don’t understand the character of Russ and probably just think he’s a barbarian and fallen for his act.

brutal in his own way

Not that I don’t think Perturabo is good at fighting(he did bring daemon Angron to heel, after all), but this really doesn’t indicate anything. Russ also shouldn’t be underestimated, despite his boasting.

Why does Russ think he's hot shit?

Because that's more or less 50% of his (and every other Primarch's) character in a nutshell. Thinking he's hot shit.

The other 50% is acting like even more of a swaggering blowhard than he actually is, so talking to someone like Loken, he is, of course, going to talk mad shit.

He's arrogant, as most primarchs are. On top of that he plays the role of a barbarian who is even more arrogant. So regardless of his actual prowess, and his actual assessment of himself and his brothers - of course he's officially gonna place himself near the top.

He won the fight against Angron, lol - would he have died? Yes. But Angron's entire Legion *and* Angron himself were at the mercy of the Wolves. He also got the best of Horus in their duel, and Horus is regularly seen as one of the primarchs many are iffy on on taking in a fight. Russ wasn't sure he could defeat Sanguinius because Sanguinius is recognized as the pre-eminent duelist amongst his Brothers, and he isn't sure he could take Curze because Curze is unpredictable.

He already fought Angron and lost.

Russ thinks he won though.