What's your definition of a killing?
This forum has 8,586 members. That doesn't mean the software has been purchased that many times, but lets just say 8,000 people have paid for ST2, and they all paid $60 bucks (I'm not sure if the price has ever been different). That comes out to $480,000. But lets keep in mind that ST2 has been in development for five years now. Nov 12 2007 was the first blog post and there's no telling how long it was in development prior to that, but we'll stick with five years. We have no way of telling how many copies have been sold in a given year, but taking that number above and dividing by five gives you $96,000 per year. I'm no tax expert, but here in the states, you can expect at least a third of that to go to the government leaving you with $64,000 per year (not taking into account any other expenses that would eat into profit). For a professional developer here in the states, that's NOT great money. It's a decent living to be sure, but with the skills that Jon obviously has, that's not great money.
Now, all that assumes that 8,000 people out of the 8,586 members have bought the software. I'm guessing it's nowhere near that number. To be fair, I don't remember the purchasing process - I suppose it's possible that you can buy ST2 and not create a forum account, which makes this whole argument pointless. But the overall point I'm trying to make is that there's not great money in this for Jon unless he's selling tens of thousands of copies.