This is standard operating procedure, mainly dealing with deleted accounts, and reducing spam or bot actions on the site. 3.9M removed over the course of 500+ videos for a global account for one of the largest countries isn't all that surprising. They also remove likes and comments for spam reasons as well, which this "article" (if you can call it that) just so happened to ignore, I can't imagine why.

YouTube prunes accounts quite often, even streamers and boy bands will have their numbers rapidly changed to deal with spam or bots. This has been normal operating procedure for over a decade.