The post had been migrated from TheoreticalPhysics.SE, and there must have been some glitch in the user account reassociation. That's why you couldn't comment initially (which was fixed later when this was brought to my attention)
Your deleted post wasn't an answer, it was a remark, which is why it wa deleted. It should have been a comment or an edit to the question. Of course, at the time, you could do neither (you'll find that you can do both now), so it's understandable that this frustrated you.
The comment thread was getting long and wasn't so constructive, which is probably why it got deleted. Your stance there was a bit aggressive as well, you seemed to be asserting that nobody but a mathematician would understand what you meant, which can be taken as offensive. This may have contributed to the reason for deletion. Please be nice on this network.
Comments are not for discussion. If you see something wrong or want to suggest an improvement, you use a comment. If a comment thread is becoming a discussion, or if you wish to hold one, you go to chat. On Stack Exchange, comments are considered third-class citizens -- can be deleted without reason. In this case, the bulk of the conversation had already been moved to chat, so there shouldn't have been an issue deleting them. Usually, comments are deleted when they can no longer serve their purpose, that is, they can no longer result in improvements to the post. This can happen if the author either incorporates the suggestions (or corrects the mistakes), or gives a refusal to do so1.
Regarding the "correction in tone" here, that was my fault: Someone had flagged the comment, and I felt that it was best to tone it down a bit.
1. There's nothing wrong with refusing changes or corrections. However, many a time it renders the comments useless, so these things get deleted.