You are aware that you see beforehand whether the game requires a separate account (at least on Steam)?

No, because as I indicated in my post, I didn’t buy it. I installed it from someone else’s account within my family group.

Also, how else would you enable crossplay/crosssave?

I don’t know, but not my concern.

And you don’t even need to use separate credentials. You can log in with Steam just fine.

Not according to the prompt I was shown.