To answer the reformulated question, I think that problems proposed in a journal do not fall under the contest problem policy. The point of those problems is not to rank solvers, it is to share interesting problems and promote discussion, which posting on MSE only helps with.
Added later: I think the biggest supporting reason for my position is that, while ...
Let's see what are the options:
Delete the answer, leave the question open.
Delete the answer, lock the question.
Don't delete the answer, lock the question.
Don't delete anything, just let it be.
Clearly (4) and (3) are opening the door for further cheating by anyone who sees the solution. And clearly (1) may very well attract more ...