Earlier quoted context omitted.
It's as central an example of mansplaining as I've ever seen; Mansplaining just means "I'm too stupid to understand your point, so I'm going to throw in a non sequitur gendered insult".
At least as I understand it, it more means explaining things to the person you're talking to as if they don't know anything about the conversation topic, even though you have no particular reason to do so. It doesn't necessarily have to be done by a man or directed at a woman. That's just how it tends to go. And obvs is a bit more fraught when it is going that way.
Leaving aside whether that's something to reasonably get upset about (how are people supposed to know exactly which facts are known to every single reader of their comment?), the way you're describing it seems pretty identical to the now-mostly-anachronistic expression of being "jewed" out of some money. The fact that the target of the slur doesn't have to be Jewish doesn't make it better; in fact, it kind of makes it worse. Hell, at least my example has its roots in a time when casual racism/sexism were accepted _pro forma_, and the term is slowly dying out. It seems to me your example is even less excusable.