Wait until people figure out how to add AI art gen + ChatGPT + Koikatu assets...
As to the question: legally, probably not. He's almost certainly 100% a clueless muppet who hasn't bought a license and is probably stealing fan-made assets to boot. Knock yourself out here and try to figure out if you can license assets legally from them:
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. From the looks of it, they're a publisher (of sorts, Japan business is weird) so there's certainly a way, legally, to do what he's doing (if you can read Japanese, have a Japanese bank account (notoriously annoying to get) and make a transnational licensing deal with them).
Given his responses on his Steam page, looks like some teenager who is in waaaay over their heads. Read the reviews, also the people giving positive reviews for free keys are utter scum bags.
TL;DR
Call me old fashioned, but stealing fan made assets & selling them on a major platform without a license is begging for the entire scene to get wasted by the NSFW version of Nintendo Lawyers. Not a fucking smart or nice thing to do. (Japan not being known for tolerating any kind of IP fan scene, fyi).