The author writes:
Addendum to #804: The axiom of choice is only excluded for the purposes of terms and conditions; you are permitted to consider systems involving the axiom of choice on the forums themselves. Since the comic itself is now such a system, it would be silly of us to exclude talking about it, wouldn't it? No, that clause is there as foreshadowing.
[Or I could replace the axiom of choice with a non-plot-critical axiom.]