Well, when you force users to login at least 1/month to keep their handle/account, you're 100% going to get an inorganic and non-truthful MAU metric, especially if you define that "A" to be <=20 seconds.
I realize the study was unable to or just did not perform “follow-ups” with the individuals they are positing “no longer work”. But, what exactly are these malcontents doing instead, then?
Living with parents? (Why would a parent enable this juvenile behavior)?
Relying on their spouse's income to survive (and, if so, is there any real harm/issue with this)?
Going “off the books” and just being part of the underground economy (in which case, this 'not working' posit is partly a falsehood).
This: https://observablehq.com/@greynoise/noise-storms has some explanation but I'd be glad to elaborate more if needed.