There is objective truth and objective reality. I’m sad that so many people reject this utterly necessary fact and spread misinformation against it.
Of course, the debate remains of the "validity" of pure mathematics for its own sake. Many mathematicians, while not being religious in the common sense, have longed for a Platonic reality. Why do you think Hardy was so derided for his Mathematician's Apology?
Mathematics is simply another formal game of language, based on a number of axioms which can be either held to be true or not. Look at the differences between Euclidean and projective geometry. No one is asking which one is "true". Projective geometry helps for some lines of thought, Euclidean works for others.
What I mean, though, is that there's only so much information that a piece of humour or a comic strip can convey.
Also, you have to consider the crowd that xkcd caters to, and the crowd that typical American geek humour such as Big Bang Theory caters to in general.
Existentialism/Nihilism are common themes in geek humour, but negatively: these ideas are quick to be dismissed as edge-lording, and as 8-chan-incelling.
Geek culture and morality relies wholly on the self-supporting idea of science as the one and only source of truth. The geek's first reaction will of course be to dismiss any claim to the contrary, in humour and otherwise.
http://www.newadvent.com/fathers/index.html
http://www.newadvent.com/summa/