> The problem is that it's easy to prove that signups are increasing, and lot harder to prove that there was a measurable increase in number of paying users.

Okay? The A/B test sought to measure which of two options A and B led to more signups.

> So, assuming that the increase in signups is driving more business is just foolhardy.

Your "A/B test enthusiast" was not testing for or trying to prove a causal relationship between increased signups and more business.

If he made the claim separately, then that is the context that is missing from now multiple comments.