If you never tell people when they are wrong, how can they learn and improve ?
I prefer accuracy over "politeness" (which, to me in the current case, sounds more like hypocrisy than actual politeness).
In the post in question you made a wrong assumption about how C2 works, I told you you were wrong and explained why you were.
I don't see the sarcasm or the lack of politeness in that.
If I just had posted "You're wrong" and did not elaborate further, I would understand and agree with your point, but I provided an explanation to elaborate.
I did not just "point finger" for the sake of pointing finger.