Ah okay, that helps me understand better. Thanks. I know that Apple collects 30% normally for IAP, which makes sense since that's their own system.
The title of the article made me think they were somehow collecting 27% from each purchase made via a third-party payment system, which was weird to me because ... at which point does that money flow through Apple's systems? That's where I was confused.
Let's see if I understand this now... so, if a developer uses a third-party payment system, then Apple doesn't get a percent of each sale (and they also wouldn't know what the user is purchasing). So instead, they charge the developer 27% of... what?