In this post I edited @fouriertransform's HINT to a complete answer. Am I allowed to do this?
One of the boilerplate reasons for rejecting an edit is
clearly conflicts with author's intent
This edit deviates from the original intent of the post. Even edits that must make drastic changes should strive to preserve the goals of the post's owner.
That is, even very substantial edits ought to preserve the intent of the post. Hence a hint-only answer shouldn't generally be edited by a third party to become a fully spelled out answer with all the details, nor vice-versa. (One exception to this, of course, is when a post is marked as community wiki.) Inclusion of the spoiler tag doesn't really affect this.
In this case, however, the edit was accepted by the original author of the answer; thus, it shouldn't be rolled back or otherwise rejected after the fact. I think that a better practice for the future might be to write your own answer, at the level of specificity you feel is appropriate, and give credit to any other answers that inspired your own.