False. The URL is currently in the Microsoft Store description.

https://www.microsoft.com/en-us/p/krita/9n6x57zgrw96

I mean in practice you can also list links to your site on the Apple App Store but you aren’t allowed to mention a cheaper price or another purchasing option.

It seems to me there is some price arbitrage opportunity for a 3rd party who can figure out how to get affiliate payments.

If the app developer can't advertise that one of their store options is cheaper, a 3rd party isn't limited by the same contractual obligation.