[qntm]

That reddit post (and admittedly it is just a reddit post) actually says the opposite: there is no theoretical basis for antimatter not to behave exactly like regular matter in the presence of a gravitational field.

The null hypothesis (and, in the absence of any strong evidence to the contrary, the stronger hypothesis) is that antimatter and matter respond to gravity in exactly the same way.

[natep]

That's what I said. The person making the bet is betting on attraction, so the response was betting on repulsion, and I responded to the latter.