So really he made his money from selling his company, not just from the game sales itself. And I would argue that he more or less got lucky more than he “earned” it, which I think he has said as much in interviews before.
I can’t really speak to if he directly exploited labor, but I think we can pretty safely state that Microsoft has in fact done so repeatedly, and so indirectly at least, Notch benefited from that as well.
Now does that make him morally corrupt for taking that offer? Maybe. But I think any one of us would take the same offer if given the chance. But the reality of the situation is that getting rich from this kind of success is very slim, and even then the labor and effort involved is very much disproportionate to what others are earning for much more effort. And if he was taxed at a rate where is was no longer a billionaire, but just a millionaire, then his quality of life very likely won’t change too much while many other people would benefit, assuming that tax money is actually going to public services, that is.