How would we go about measuring this inequality? With standardized testing—or indeed any testing that measures aptitude in actual math—being phased out, what do you propose be used as the measure of math aptitude that could inform us of changes in inequality in math proficiency?

However the commenter likes. I think it's ridiculous to say "I guarantee this will increase inequality". But if he's that confident... and no doubt he is a "he"... let's put money where mouths are.

It could be entirely reasonable to guarantee something like that. It's a figure of speech as well, and doesn't imply that they have a way of proving it. And why are you confident they are a "he"?

Anyway, I think the onus is on you to specify a way of settling the bet since you offered it.