If the minimum guarantee threshold is higher than the writer's current typical monthly total, as it is for me right now, then it is worth it. It doesn't cost Medium to guarantee that a writer will earn what they would anyway, but it wouldn't incentivize either.

It sounds to me as though the author was making over $500 per month with a bonus, a bonus that Medium is currently not paying. So, if he were incentivized by the money (which he says he isn't), then the $500 minimum guarantee certainly would be incentivizing, as I'm sure the bonus was.

Bonuses and minimum guarantees will probably lead to more consistent writing from the more skillful writers, even if they don't need the money (as I don't). I do like to be paid for my effort though, and Medium has paid me very well over the years.