Sounds like pretty standard Apple App Store “reviewer roulette” to me. Each reviewer seems to read Apple’s rules differently and consistency is not clearly enforced.

You don't subscribe to Steam Link, however. It's a "free" service.

I bet if Google or Microsoft were willing to give Apple a cut of the subscription cost for Stadia/GamePass by using IAP, there wouldn't be a story here.

I guess Steam Link is treated more like VNC/RDP client for your gaming PC.