No, he’s not fought any kaiju in the ‘alien bioweapon’ sense. Meanwhile they have fought each other and know how to fight kaiju, as well as armed smart humanoids. Heck, before leaving skull island and fighting G and Mecha G he’d only fought skullcrawlers- not even close to comparable! The physical anatomy of Raijin is not particularly similar to his foes. They’ve fought things more similar to him than vice versa, and just have far more combats period.
Whip tails, absorption, electric bite, he’s faced none of these abilities before. They have faced blades wielded by bipeds before.
And again, just looking how they *actually fight*. Kong is talented but sloppy and makes a lot of mistakes like getting his axe stuck, ignoring Godzilla to focus on getting the axe out, just standing and eating the occasional frontal charge he could’ve dodged, and the kaiju do have some nice skill feats and various tactics and there’s two of them so if *he* makes a mistake, either can exploit it, while if one of them do, the other can cover.
His punches and blows really aren’t anything out of line with PR. Raijin’s charged slap especially will not have trouble competing, but even a shoulder check sent a jaeger through 3 buildings casually- these are strong kaiju. Cat 5s aren’t like cat 4s, they’re a very solid physical step above everything else in PR, which is why so much effort was put into keeping Raijin at a distance.