Edit: I have read in detail https://developer.apple.com/app-store/review/guidelines/#payments , mainly sections 3.1.3:
3.1.3(b) Multiplatform Services: Apps that operate across multiple platforms may allow users to access content, subscriptions, or features they have acquired elsewhere, including consumable items in multi-platform games, provided those items are also available as in-app purchases within the app. You must not directly or indirectly target iOS users to use a purchasing method other than in-app purchase, and your general communications about other purchasing methods must not discourage use of in-app purchase.
The main issue for me is the sentence "provided those items are also available as in-app purchases within the app". Does this refer to the multi-platform game consumable items only? If yes, to my understanding I do not need to offer the services as in-app purchase, provided the service is multi-platform.
Because of the ","s surrounding the phrase "including consumable items in multi-platform games" the word "items" in the phrase that follows it, namely "provided those items...", refers to the broad list of items, specifically "content, subscriptions, or features, including consumable items", not the phrase within the commas.
So the answer is - you must include in the app an option to make the purchase via IAP.