It's based on a survey with ~400 responses, and 50% of the responders are in some way affiliated with the surveyer (no information about any methodology is available, it seems more of a marketing stunt than a "serious" survey of the landscape)
Yeah that sounds very unlikely to me. I guess the definition of "open source maintainer" and "unpaid" are wide open. Does it count if a company pays your salary and you are allowed to do the open source stuff as part of your job? How popular does your project have to be before you are an "open source maintainer"?
I think lots of the kernel folks are paid to contribute full time. For a while I was paid a full time maintainer on some apache licensed search stuff. Before that web stuff.