I think you are referring to this question.
What you are saying about the problem is interesting, but I am not sure that was the what the OP was asking, i.e. although there is a research-level answer to the question as you have explained above, the way the question was phrased by the OP didn't seem to be asking for a research level question, it seemed that the user was not aware of standard material on finding all sources shortest path (and honestly it still seems so to me, e.g. "I'm aware that the single source shortest path in a undirected and unweighted graph can be easily solved by BFS.", or the comment posted by OP as reply to MCH's comment), therefore it seemed to be about material covered in typical undergraduate algorithms courses, and thus off-topic for cstheory.
The question had a close vote and another upvoted comment that this is covered in standard textbooks in algorithms when I migrated it to math.SE. It had already one answer posted, since there was disagreement whether we should delete the answers for non-research level question and the preferred solution IIRC was migrating the answer and the question to Math.SE I migrated it to Math.SE in place of simply closing it.
This is the explanation for why it got migrated to Math.SE.
I would be happy to hear any suggestions on how I can improve my decision making when closing/imigrating questions (it might be better to open a new discussion).
Another issue is should we reopen it and erase the migration history or not. I personally lean towards reopening it because I think the research level answer that you have explained should be made available on the main site although IMO it is not what the OP was asking for. I think we can also edit the question to make it look research level.