The code is open, it's easy to check who's contributing what:

https://github.com/mdn/content/pulse/monthly

tl;dr: a slight majority of the recent top contributors do not appear to be Mozilla employees: I'm seeing Yale, several freelancers (paid by Mozilla?), and a bunch of random companies.

Hi, I'm part of the MDN team, and you're absolutely right! We’re fortunate to have a wide network of contributors, contractors, and partners who help us keep MDN accurate and up to date. As an open-source project, we deeply value and appreciate the incredible support we receive in building and maintaining extensive MDN content.