They are talking about the cost to run the Actions control plane and the scheduler that is not executed on the runner itself.

They have all kinds of costs hosting GitHub, which is why there's per seat pricing for companies. If those prices are too low, they can always increase them. Charging on top of that per minute of using your own infrastructure felt greedy to me. And the fact that this was supposed to be tied to one of the lesser-maintained features of GitHub raised eyebrows on top of that.