It doesn't say that anywhere - there are 0% payment fees on transactions in Ghost.

No. Ghost.org provides a hosting service, that makes 4 million dollars a year.

Users of Ghost (whether on Ghost.org, or self hosted) who have set up payments, bring in 10 million dollars a year to themselves, in total.

So, 4 dollars out of less than 10 total, then (subtracting out self hosted sites' revenue)?

This part of the copy confused me too.

What percentage of ghost users set up payments at all? I'm guessing it is a tiny fraction, making the ratio of the two numbers meaningless.

(Independently, the two numbers are each a great achievement!)

The two numbers are entirely unrelated. Ghost does not take 40% of your earnings. If you take a platform such as Wordpress, you can pay $5 every month to have it hosted, and eventually set up a payment system on top. It's not Wordpress taking a cut of your payments, it's you paying them for a service you would have to pay them for anyways, payments setup or not.

Same thing for Ghost.