This is sarcastic, in fact, you are the one who didn't get it, sorry. And by the way, the correct sentence shoud have been :
"I'm not sure you understand sarcasm : furthermore, I guess English isn't your mother tongue ?"
And certainly not this approximation :
I'm a bit fed up with this "not your mother tongue" remark as I do write a way far better English than most of the natives on this website. Period.