Actually encouraging a free software application (it's GPL) to target more closed platforms in order to add in-app purchases for basic functionality is very cynic-sounding to me ... am I just being old?
I think you are misreading the intent there a bit.
I saw the suggestion as taking payment for a physical printing service (where there are actual costs to cover: materials, time, delivery) - not taking payment for core/basic features of the software.
As long as submitting to a local printer (which would be a core feature, if printing support is present), or exporting to a format that can be imported into something else for local printing, is not charged for, all would be well.
Anyone could then publish a free clone though? Anyway, locking user's data in the app and then charging the user for getting it out is definitely against the spirit of FSF who publishes GPL.
He said "A free app with in-app purchases to export the result for 3d printing." Do you expect him to add an "export the result for 3d printing" feature without a payment model?
I am apparently being misunderstood. I don't mean to imply that including a feature for exporting models is a bad idea. But I don't understand why it's controversial to take advantage of platform's payment service for actually printing those models as physical objects and sending them to users. Obviously one would not do that for free.