In actual fact, this is entirely my fault. A user had flagged the question indicating that it was quite closely related to a problem in a KöMaL competition that ended yesterday. When I first saw it, the question and the answer were deleted, and I just let it stand. This (my-)morning I undeleted the question since it seemed to have received an answer and appeared to be another see-answer-and-run situation, and was then curious why you had deleted your answer.
What I should have written is something along these lines:
The question above is closely related to a problem in a KöMaL competition. If this is the reason you decided to delete your answer, I just want to let you know that the competition has now finished, so feel free to undelete it.
(I also should have had more coffee this morning; working on that presently.)
In general there is no good way to respond to moderator comments on deleted posts. That regular users cannot comment on deleted posts is certainly status-by-design. Raising flags in these instances is not really appropriate (though I guess could work in a pinch). You could comment on some other undeleted post of my own (again, slight misuse and I don't want to receive a ton of notifications in this manner, but as a mod I can clear out the off-topic stuff rather quickly). I'm personally an infrequent guest in the main chatroom (though other mods are fairly active, and they can reach me fairly easily if I'm available; maybe I should try to ensure that I'm at least pingable from the main room by popping my
head avatar in there every couple days).
Again, my comment was unclear and therefore bad, and I apologise for any confusion.