lbschenkel I now managed to replicate your findings. I installed the app in a completely new user profile and tried to activate it. Went through all the steps and in the final one, I got the message you're referring to.
So it looks like the app is doing different — and more stringent — checks at activation time than at launch time. So if you are like me and have a profile already activated, the app continues to work but apparently you can no longer activate it for new installations.
I'm not sure exactly what the app is looking for. If it's Play Integrity, then it's game over.
With that said, perhaps this check is only for the passport/ID scan flow and if you manage to activate your MitID via an alternative way (such as a visit to the citizen center), the app perhaps can still work. That will need to be investigated.
I'll put a prominent notice in the gist about this.