Is it specific to one project only? Or do you see that on multiple projects?
When devs are uploading their game, they have the option to give it a different “noun”, so instead of calling it a game, Itch will call it something different.
I suspect a dev marked a game as an “experience” instead of a game, and Itch then displayed that on a button, so instead of saying “Add game to collection” it says “Add experience to collection.
This is just a guess, I can’t think of anything else.