I don't think that's an accurate comparison. Medium is an affiliate payment. In essence if you refer a customer to them, you get 50%. No different than Amazon affiliates, except Bezos pays 5%. lol.

Substack is not an affiliate gig. At Substack, you're selling a subscription to your content only. If people sign up and pay, they get nothing else, just your emails. Substack keeps 10% for managing that and doing the billing.