I agree with that sentiment on the whole. But in this case I don't see that anything at all has been added: it was just moved from a solution into the problem. Additionally something has been _lost_, that is, the original context which was self-contained and didn't have significant flaws.
Allowing posts to change so much makes me fearful for the integrity solutions tied to the questions. It seems like there is a good chance of unintended problems, hence my feeling that good editing should be conservative.