If someone seems to have productivity gains when using an AI, it is hard to come up with an alternate explanation for why they did.

If someone sees no productivity gains when using an AI (or a productivity decrease), it is easy to come up with ways it might have happened that weren't related to the AI.

This is an inherent imbalance in the claims, even if we both people have brought 100% proof of there specific claims.

A single instance of something doing X is proof of the claim that something can do X, but no amount of instances of something not doing X is proof of the claim that something cannot do X. (Note, this is different from people claiming that something always does X, as one counter example is enough to disprove that.)

Same issue in math with the difference between proving a conjecture is sometimes true and proving it is never true. Only one of these can be proven by examples (and only a single example is needed). The other can't be proven even by millions of examples.