As mentioned in the comments, the scope of this site is completely independent of the scope of any other SE site. That implies:
- The fact that a given question might be on-topic elsewhere is entirely irrelevant to whether it is on-topic here.
- The fact that a given question might be off-topic elsewhere is entirely irrelevant to whether it is on-topic here.
In other words, considerations about whether a given question would or would not be acceptable on other sites do not play a role in whether it should be closed if it's posted here.
Migration is a subset of closure: we only migrate things which are off-topic here, and that decision is independent of whether they are on-topic elsewhere; that only comes into the process when deciding whether the appropriate action is a straight closure or a migration. If a question is on-topic here, it shouldn't be migrated, no matter how on-topic it might be elsewhere.
Now, that said:
- If a question has been closed and you feel it might be well received elsewhere on the SE network, you're free to suggest to OP that they flag it for migration.
If a question is on-topic but you feel that it would be better received elsewhere on the SE network, and that OP might not be aware of the possibility, then it is perfectly acceptable to suggest in a comment something like
would X be a better home for this question?
and it is then up to OP whether to flag for migration, or (with due regard to the network etiquette) to cross-post a re-tailored version of the question on the other site.
The things that you are not allowed to do in comments are spelled out in the Code of Conduct. So long as you conduct yourself politely, there's nothing stopping you from providing such suggestions.