Somebody argued to me that it is to support editing like source code where you want a line length limit like 80 characters.

That is a very specific thing for source code editors. But bad for comment boxes. So I don't understand the popularity of markdown for comment boxes.

IMHO markdown should have done the opposite, make listening to all your newlines the default, and if you want to make it ignore a newline, end the line with a backslash or something like that.