He has Over 358,000 subscribers on SubStack.
I have been told by SubStack support that average subscribers that convert to paying is 10%, lets be conservative and say 6%, that would equal a little more than 20k paying per month
20,000 x $5 per month = $100k
Twitter is asking him to forfeit that much income to use Twitter instead.
That is going to be a tough sell for SubStack publishers that have even a small amount of paid followers.