I read the "Terms of Use"/various documents Apple makes you read when joining the "Community Support" service today and it still had the "This solved my question" wording and nothing regarding a specific etiquette/bad form.
Here is one of the links shown during the sign up process that I still had open on Safari:
Solved, Helpful, and Apple recommended posts - Apple Support
On the same page they use the wording "best answer" once while referring to the answer that solved the question.
Maybe the page/document linked above still has to be updated by Apple (?) but even if this is the case this should not make the user jointly liable for following the guidelines stated in the page/document presented to the user during the sign up process.
I believe it's a matter of perspective. If the wording is "This solved my question" it makes sense to remind the OP to mark the question as answered for the sake of simplicity and ease of use for future readers (as Apple encourages the user to do on the page linked above). On the other hand if "best answer" was the only option available I can see how "reminding" the OP to mark the answer as such can be seen as "bad form".
All considered I don't believe my answer was rightfully edited by the moderator in this case. I'm here to give my contribution, help others and find answers all while being part of this community. If moderators can claim the right to go around and edit answers even when there is not a real reason to do so (like in this case) then I don't think this website can be a functional service to the community.
To the OP: sorry for the (way too) long off topic but the situation had to be addressed. I'm glad you found the answer to your question.
Peace
sc