You can look any youtuber up on socialblade and see their earnings range. It's not small which goes to show his diversified stuff must make a shitload.
I'm not a game Dev but our platforms I work with have corporate and learning editions. I have my work email registered with our enterprise install and my personal email for the learning. They could potentially cross reference and find me using both.
In this instance though a smaller game Dev would do a lot of BYOD and I think this comes down to business processes to make sure staff do not register using work emails unless it is directly for work purposes.
Unity also should be flagging it to them and terminating the personal accounts imho. But that being said how does Unity expect developers to upskill in their tooling in their personal time if they won't let them practice. You can't expect a company to pay for a pro license for someone not employed to work on that stack.
Audiobookshelf and LL are it. Or calibre automation but it sucks.