I'm here to answer this question! I'm Medium's Director of Publisher Growth, and I issue the contracts to editors participating in the program. They are paid monthly for their Boost nominations that are approved.

This program is still in beta, because we're learning a LOT behind the scenes about how to build out the tools that will eventually scale. But I can confirm that participating Boosters are indeed being paid monthly, and have been since March.

I have a post coming on the Medium Blog (formerly known as 3MR) soon, but in the meantime I'm happy to answer questions!