I think that even a supplemental answer should be ... an answer. If it reads more as an extended comment, this is problematic (which is not the case with your example).
My preference is to link to the answer rather than referencing the user name of the poster (although this is not wrong, I am merely stating my opinion), because it helps others find it and prevents this site from looking like a forum rather than a Q&A site. The other reasons are:
- Someone could edit or delete their answer. The dead link will at least serve as notice for a deleted answer, but if you didn't cite at least a portion of the answer and it is later removed and/or radically changed, the reader may not be able to follow your response.
- The order of answers changes based on voting. Your (supplemental) answer might end up having more upvotes than the answer you are expanding on, and yours may even be accepted as the top answer. Will this confuse readers if your argument is not self-contained?
- As nice as it would be for folks who find us on Google to read both your answer and the one you're supplementing, most won't. It is more helpful to the Internet at large to provide enough of a summary to ensure your reasoning can be followed if they only read your post. Perhaps they will only get one aspect of the explanation, but that is fine so long as it provides an answer.
On the other hand:
- There is no point in duplicating existing content with which you agree. A quick one-or-two-sentence summary or representative citation is perfectly acceptable.
- Adding more information than exists in current answers makes the Internet a better place. It is better to err on the side of providing a quality response than to withhold information because of the effort involved (hopefully merely citing the thesis statement or conclusion of the existing answer makes the bar low enough as to not dissuade you and others from posting supplemental answers).
So I would say go for it. Feel free to post supplemental answers. Just make sure they are indeed answers.1
1 Your current answer does an excellent job at being an actual answer, so it is fine as is. My only preference would be that you also link to Niobius' answer, not just reference it by his username alone. In addition to helping the site remain focused more as a Q&A site rather than a forum, Niobius could actually post more than one answer to a question (maybe because he wants to play devil's advocate). It's rare, but if this happens, how will readers know which of his answers you are referring to?