When a question is migrated and then closed or deleted on the target site, the migration is considered "rejected." That causes the target site's copy to be locked and the source site's copy to be unlocked but left closed with the generic off-topic reason. (See the timeline here and on Server Fault.)
The rationale for immediately locking closed migrated questions is that questions in need of improvement shouldn't be migrated because that dumps one site's problem on another's community, which is rude. In this case, though, it's debatable whether the question needs any improvement before it's acceptable here. I agree with you in that it's not primarily subjective/opinion-based. Answers to it can cite facts.
Leaving it in this state would be a shame, since the answers are good, while the current unlocked copy on Server Fault has no answers. (Therefore, re-migrating it would give us a version without the answers.) The question is clearly off-topic on Server Fault - so there's no danger of it being reopened there and duplicating the communities' efforts - while it's on-topic here. It would be great if a Super User moderator unlocked it. Conveniently, doing that would automatically change the migration notice from the "rejected" one to a normal one.
Relevant MSE: Treat question migrations better for almost-fits-but-not-quite questions
