Uhm. I do not really understand what exactly the problem was or is on that site. (And I do not care)
https://sketchfab.com/tags/mp5
Also there is lots of weapons and models of brand items and characters on that site.
Viewing post in "Copyright" or "Transformative", "Fair Use"?
Now if only we knew, if that was empty threats or if they had a case. Whoever sent those evil letters. I assume they got evil letters.
I only spent a few minutes on that site, and you can't scroll for a second without seeing something that is from a known ip. Including stuff from Disney. But I looked longer on the H&K things.
And there might be another aspect about all this. Maybe you could have the implement in your game, no problem. But offering the model of the implement might be problematic.
They have their faults and quirks, but this is not the impression I got from Itch. Maybe cool down a bit and read what really happened and what was really said. And consider that Itch is not an advertisement platform. They do only make money from sales, and only when the developer adjusted their revenue share. So they can't pay you for downloads and traffic and site views, as some other sites might be able to. You can switch to direct payments and deal with all those things yourself, if you make sales.