I think the point was that it's inherently less safe to allow arbitrary markup and then attempt to sanitize it, than to make a full parser that's incapable of generating unsafe HTML at any stage, all other things being equal.
The safety of widely-deployed Markdown + sanitizer libraries is largely thanks to testing at scale and a history of patches for XSS vulnerabilities.
A) It was not as simple as you suggested if there was markup involved in the message.
B) They'd have to use a parser and I linked to a parser that sanitizes that was once used in a pretty big network of sites.
I'm uncertain if you misunderstood or are simply agreeing with me in a tone of writing that makes it sound like you disagree.