Hundreds of thousands of dollars to create in app subscriptions? There are many indy developers that have done that as a one man operation.
Right now there's no mobile-first onboarding flow. That's a completely new thing they'll have to build. They'll also need to update all of their code that checks subscriptions, handles dunning, etc., to now work with multiple billing providers. So yeah, it's going to take a lot of time, which is a lot of dollars.
Both valid perspectives IMO.
They obviously knew that Apple Pay subscriptions were an option when they were whiteboarding the product, and they decided to not do them. That says to me that they weighed the options, did some math, and decided that the numbers worked better if they didn't spend time on that, and kept 100% of the money.