I don't think that is correct. Both use Stripe, yes. But if someone's receipt says Medium, that means the writer did not set up a separate account for Substack, or has the wrong one connected.

I currently have 3 different Stripe "businesses". One for Medium, one for Substack, and one for Ko-fi. If you have the wrong one connected, the receipt will be that of the incorrect business.

I had this happen accidentally once when someone donated to my Ko-fi and their receipt said Medium.

Hope that makes sense. Stripe is rather complicated to use.