Wouldn’t “not enough to sustain the business” be failing? I’d think “mediocre success” would be doing well enough that you can’t just throw in the towel, but not well enough that you can escape the grind.
The author defined mediocre success to be the relative outcome of an experiment, such as an A-B test, he wasn’t talking about income levels or overall success. It might be assumed that, since we are talking about startups and experiments, that we are not discussing self-sustaining businesses? Either way, which interpretation makes the rest of the article make the most sense?