"ROFL NO UR NOT I DOUBT THAT ROFL."
If that is verbatim, it was sarcasm.
If that is a paraphrase and said in sincerity, then I can only speculate that it is the hyperactive maladjusted ejaculate of an under-socialized manchild, who is so underexposed to women in real life that he cannot help but fawn all over himself in incredulity at the opportunity to converse with one over THE INTERNETS.
Also, it
is a type of sexism, just not a traditional one based on gender roles and whatnot. "Sexism" can also mean discrimination, prejudice, or special treatment based solely on gender, something that you can't really deny happens on the internet, where women are put on a pedestal by some people.
Fake edit: oh yeah, and like Jester said, because there actually are a lot of weirdos who masquerade as women on the internet, for one reason or another, despite actually being male.