A lot of the top writers have large guaranteed incomes and related incentive structures. The 20M is not all revenue, some of it is a cost for Substack.
I couldn’t reply sooner because I have noprocast on but my numbers were a bit off, I was repeating what I heard on a tech podcast. According to the numbers they had to release it was a bit higher https://www.axios.com/2023/04/07/substack-gets-writers-to-in...
(I don’t think you’re wrong by a factor of 10, just curious where you got your number from.)