I don't see the problem with doing your job super efficiently. Adding bugs is just "a duck", not a real productivity loss.
I don't see how you can claim the company wants hours rather than work done.
Aside from that I'm not sure I buy this "duty of loyalty" idea. What social purpose does it serve? Sounds like something that's common because at some point it became normal to be able to get people to agree to it. After all, most people in history could not work remotely.
There's also plenty of other professions where you can work for several people at once, for instance you can be a columnist and do prep work that's useful for more than one column.
What I don't like is this idea that he's being paid for his time, not his product. It seems pretty clear cut he's being paid to maintain this legacy system. He's doing it. If something happens, he should be available, but otherwise he should be able to do whatever he wants, including other work.