Can someone enlighten me? I honestly don't get what it is or what it's for. Surely OpenRouter knows who the providers are?

Yeah these stealth models pop up from time to time. Openrouter knows, but doesn't share. Users can use a testing version of something for free and in return the provider generally is allowed to retain the prompts sent in to get real world use. In the past I only really remember using one that was surprisingly good, and then it turned out to be GLM-5.1, speculating on what one this ends up being is part of the fun.