He's doing VASTLY better than e.g. if he had taken that book as a first time author to a technical publisher. For one, the book is actually published: he'd still have months on the timeline at even the most progressive dead tree publishers. For another, at similar sales levels, his ~$5,000 advance would not be "earned out" yet and he could expect his first royalty check the day after never.
With regards to authoring as a commercially viable profession: traditionally, it isn't for the majority of published authors. There do exist folks who make incomes engineers would find reasonable from selling snooks (including technical ebooks). Well, OK, some of them make incomes engineers would find unreasonable, but not for being too low. (Independent authoring, like independenr software development, is 10% about writing and 90% about successful execution of a sells-writing business.)