They aim at different markets. Substack is for writers that already have a following, it is more like a SaaS. Medium is for upstarts trying to gain a following through the platform, it is more like a social network.
It's effectively a blog generator with a built in payment button. A lot of the success of Substack hinges on the fact that Twitter exists. It's the social media funnel to the site where most of the actual value is.