The editor describes it as "moved the self-answer to a real answer." Basically, the OP had edited their question to include a solution they had discovered for their self. The editor has extracted the edit and pasted the text into an answer; at the time of writing it is the only response in the thread.
On the one hand this is the proper Stack Overflow mechanism, because answers can be voted on and accepted. On the other hand, the editor stands to gain the reputation, rather than the OP who provided the actual solution.
I have rejected the edit because I don't want the editor gaining points for the solution provided by the OP, but still the answer — containing the text from the edited question — stands, and can be voted on. Presumably the OP won't accept, but even so.
I know the precisely correct answer is "flag it and let the moderators sort it out," but I think it's good to have policy explicitly defined in Meta.