According to the question timeline:
The question was asked on Physics on 2015-05-24.
On Physics, it was answered by Jimmy360, docscience, anna v, WhatRoughBeast, Loren Pechtel, Thorsten S., Cedric H., Thomas Pornin.
It was migrated from Physics to Worldbuilding on 2015-05-26. Note: Migrating a question with that many existing answers is now considered a terrible idea. That probably wasn't as clear in 2015.
On Worldbuilding, it was answered by iNeedToMakeBetterQuestions, Paul Krueger, Nick, user15361, cybernard, colmde.
30 days after the migration, the original question was automatically deleted on Physics. (That's how migration works.)
On 2017-10-17 the question here received a close vote, entered review, and was closed by the community.
The migration (remember that? From 2015?) was rejected. When a migration is rejected, what's supposed to happen is that the migrated answers are deleted and locked on the receiving site and undeleted on the original question -- it sends the received content back to the originating site to handle. But the original question was already deleted. Probably what this means (I don't have 10k rep on Physics to check) is that the question there is still deleted but, within that scope, the answers are not deleted -- that is, were the Physics question to be undeleted, the answers would return there. That's a guess.
I don't know what's supposed to happen to answers from the new site when a migration is rejected. I think the idea is: we keep ours, they get theirs back -- but we don't send ours back across the migration line to the other site. The SE code avoids having an answer be live on both sides automatically, so because the WB answers didn't go to Physics, they stayed here undeleted.
Have I mentioned recently how freaking confusing migrations can be?
I think one part of what I've said here should be treated as a bug: if the rejection fails (because the other site has already deleted the original question), then it shouldn't delete anything on the receiving site. (And it certainly shouldn't lock anything, which prevents community undeletion.) I have unlockd and undeleted the answers that were deleted by the system. (One was deleted from review, which I won't undelete.) I've also made a feature request to not reject old migrations.
My undeletions restore the status quo. I didn't evaluate those answers for quality. Some are pretty short, so maybe they need more attention from the community.