Adjuncts apparently get paid pretty poorly. But the fact that it's hard to make a living as an adjunct professor/lecturer doesn't change the fact that it's at least as hard to make a living doing online courses.
What I’m saying is that we generally don’t expect that part-time jobs get paid as much, in total, as full-time jobs. Someone working 1/4 time getting paid 1/4 what a full time person gets (maybe a little more - or a little less - for the flexibility and/or privilege) is normal.
So asking what fraction of their time Udemy instructors spend on Udemy tasks seems like a prerequisite for judging the fairness of their pay, and I don’t think this article covers that.
But I would assume a month of work would probably cover most new course content (and could be less)--given my experience in putting presentations together for conferences.
That said, given that assumption which I think is at least somewhere in the ballpark, typical revenues most seem to get don't really move the needle except as a very part-time hobby. Once upon a time I had a shareware business which brought in $7K/year for a bit but that only made financial sense in that it was something I enjoyed doing for a time and I walked away once I was sort of tired of it.
Both those numbers took me less than a minute to find.
[0] https://www.bls.gov/ooh/Education-Training-and-Library/Posts...