You have the reputation required to vote to close questions as off topic or to have them migrated to a different site. When you flag to have something migrated, you're using the tools you're given the wrong way.
Whether a question is on topic on this site is completely independent of its quality. The question you linked to is, IMO, not low quality. Low quality means:
This question has severe formatting or content problems. This question is unlikely to be salvageable through editing, and might need to be removed.
None of this is the case. The question's formatting is very good, and its content is substantial and easy to understand. It's not on topic on this site, correct, but your flag was still wrong.