I'm going to set aside the issues with the linked question and explain, very briefly, the modus operandi we generally adhere to.
We should only decide whether a question is on topic or not. Just because a question is also on topic on another site is not a ground to migrate it, even if it was better suited there. If you think a question would be fine here, and find an answer, then it should stay.
A question is not automatically off topic here, just because it is on topic somewhere else.
There is nothing wrong saying that someone would maybe find a better answer somewhere else. However, suggesting migration is then not the path to choose. Better is deleting and asking again. Cross posts usually get closed and when suggesting this action, one should also mention this.
One caveat is that you may ask a question on different sites, if you want to focus on different things. But then you should explicitly mention this and preferably not ask at the same time.
Migration in general is a thing we prefer not to do. Sometimes a good question simply gets posted at the wrong site; it should be closed first to determine that it is off topic, then flagged for migration. In borderline cases we moderators have to investigate if it is on topic at the target site, because migrations can be rejected and we don't want to give more work to other moderators. We do not have migration paths, because we hardly ever use this feature in the first place. There simply is not enough data, to support setting these up.
When in doubt use the custom flag to contact one (and all) of us ("... in need of moderator intervention").