Related, but old and not on GD: Is game development no longer relevant on SO due to the creation of Game Dev SE?

I'm unclear when, or even if we should be trying to migrate on-topic questions from StackOverflow to GameDev.SE. Migrating on-topic questions, that game developers are more equipped to answer than other programmers, is good for the ecosystem here and advertises the existence of this site.

When I tried to migrate this question, I got in to a bit of a comment war: How to know if the player is signed in?. The OP eventually cross-posted and got 8 votes, and an accepted answer with 7 votes: How to know if the player is signed in?

I also flagged this for migration: https://stackoverflow.com/questions/17455038/parallax-background-resizing. But the migration was declined: "This isn't off-topic here".

If that's a reasonable rejection reason, is there ever a case where a question is off-topic on SO but on-topic here?

Where is the line?