I can't see how this is an argument against the existence or knowledge of absolute truth.
All it says (very nicely by the way) is that there are many things we don't know or don't realise, and lots of amazing things exist.
It is still absolutely true in mathematics that 2+2=4 for example*, although I would agree that we might have to be careful or closely examine our assumptions before claiming something is absolutely true/false.
* (to any pedants or mathematicians: yes I know that this is technically not always true, but 2+2 can always be shown to equal the same thing that 4 equals, so it is actually true)
I made a copy of this. I think it's a great response to anyone who claims they "know the truth" about existence. And since I live in a very religious area, these are almost always theists. It demonstrates that what we experience is really just a tiny bit of "reality." Conversely, however, this is also a great response to someone who claims any truth. I guess, as cognizant organisms who ponder their own existence, humans can't be sure of anything.