I was the moderator who deleted the comment after it was flagged.
My thought process was as follows.
- A close inspection of the question made a couple things clear. The original version of the question (to which the comment was posted) consisted of two parts: the question (which was devoid of any mathematical typesetting), and a "proof" which seemed to include quite a bit of mathematical formatting.
- Before this meta-thread began, I had to question what the target of the comment was.
- If it was the plain-text question, then some other user had subsequently edited that so that it was at least using standard notations. As I could not see how this part would still be "horribly written", the comment would then be obsolete in this case.
- If it was the proof (which I now know to be the case), then I noticed that it was simply an image taken from some other source. If that part was "horribly written" one can hardly blame the OP for it. And simply pointing out that to you it was "horribly written and [didn't] even make sense" wasn't going to help anyone answer the question. This started to feel like shades of "not constructive". Furthermore, as there was then a perfectly suitable answer which also addressed the (to me possible) concern of the proof being "horribly written", I could also judge the comment to be obsolete.
- Since everything I thought of lead to the conclusion that the comment should be deleted, I deleted the comment.
You are absolutely right that there are many (thousands) of questionable comments out there that probably ought to be deleted. I don't go around looking for these, and I doubt any moderator does. But when they are brought to our attention via flags we do look into them and make a decision. And, speaking for myself only though I'm sure the other moderators would concur, it is never a robotic reflex to just delete them.
As an added note, my decision to delete had nothing whatsoever to do with you, Git Gud. If the author of the comment turned out to be Asaf or Did or Georges Elencwajg I would have made the same decision. I honestly have a quite good opinion of you, though it appears we have found ourselves on opposite sides of such a meta-thread a couple of times now.
Finally, I welcome you (or anyone else) to flag any comments of mine that are deemed unworthy of keeping. I now try to delete them when they have served their purpose, but I fully admit that there are certainly some obsolete, or chatty, or even non-constructive comments of mine still floating around that would be best deleted.