Until a court order stopped them, Apple was collecting a 27% tax on certain external payments even though they didn't control the payment rails. They required developers to report their external payment revenue and sent them invoices. Developers had to commit to that or their apps would be rejected for having external payments.
If usage of an app gives Apple some justification for taxing payments, by similar logic would usage of the iPhone itself, and Safari, give them a similar some justification?
"The user would have used our payment rails had there not been other options" seems to apply universally; Apple could say the same thing about website owners steering users away from some expensive "Apple web pay" option.
I think the difference is just leverage. Apple isn't curating what websites iOS users are allowed to visit (yet...), so they can't tell website owners "pay up or we'll block you".
It's at least "reasonable" that if the app was where users derived usage, and would've purchased thru the app but for the external purchasing option, then apple has a case for it.
However, there's no such case for web (as in, web _only_).