Their argument doesn’t even make sense. No one is receiving digital content or services from creators. The content is publicly available for anyone to be seen and not purchased in exchange of a tip. This is not an in-app purchase.
Discussion
If Apple doesn’t want any transfer of value to take place outside of their in-app purchase then they have to make it clear but here they are just bending the nature of zaps to fit their definition of « in-app purchase ».