The feature is that it allows you take payments from anyone by having them enter their CC, and deposit the funds as directed by your Stripe acct. Clearly not in-app purchases. The functionality is similar to Square, without the swipe.

Indeed.

Another app though? wouldn't you lose brand equity and an upgrade path to the existing users?

I don't think I'd characterize it as "massively risky" - it took him about a month to develop.