This is a blatant attempt by Apple to extra their 30% rent on business that happens outside of their App Store. It's shocking that one of the wealthiest companies in the world acts this way towards small developers and companies.
It’s not shocking at all. Corporations pull this kind of shit constantly.
And if the only way to get in-game coins was by buying stickers off-app Apple would say you have to offer buying them in-app too as an IAP.
Like the basic tenant of “we take a 30% cut of the sale of all digital goods on our platform” is pretty straightforward, you only run into weird rules when you try find ways around it.
You might be able to get away with skirting this rule if your white-label app is just a client to a fitness tracking service. Then it should fall under the same rules as Netflix and other "reader" apps.