Yes, it is just me. Chris and a few others help a lot on the forum, and with the wiki, but everything else is just me.
The reason I can do this is, well, for one I'm a pretty good programmer, but I think more importantly, I have a very strong focus on quality and clarity. I touched on this on another topic:
Basically, if there are problems in Keyboard Maestro that result in me having to answer a dozen support emails a day, then that is time taken away from development on Keyboard Maestro, not to mention unpleasant time for my customers. So I ensure my customers don't get the same problems over and over again by adjusting Keyboard Maestro to remove or clarify the problems. This makes Apple’s crappy buggy security preference system all the more annoying because I cannot do anything to fix it.
Similarly, there are almost no crashes in Keyboard Maestro (my current estimate is about 1 crash per 50 user-years). This stems from me asking for every crash, not just how the crash could be fixed, but how the crash could have been avoided in the first place.
I also have a strong belief in orthogonality of features, by which I mean each feature should be independent of other features, and should combine and complement them. The wiki home page has a list of the main ones, things like triggers, actions, functions, tokens, conditions. You don't have to know anything about actions to understand the triggers. You don't have to know anything about tokens to understand functions. But you can use tokens and functions together to add power to each other. I also tend to make my actions simpler and resist adding facilities to an action that can be built with other actions. People coming from Quickeys tended to hit this and find Keyboard Maestro to be more complex because they had to use two or three actions to make one Quickeys action, but it actually makes it simpler because each action itself is simpler, and also more powerful, because the actions can be combined in different ways.
And then, as Chris alludes to, I've been developing Keyboard Maestro for almost 20 years now, slowly building and shaping it. I compare programming to whittling in the sense that the end result is caused by thousands of tiny changes. It's important to ensure those changes work together to make something harmonious, and not something that looks patched together (which is my way of apologising to those folks who have made perfectly reasonable suggestions over the years that I have not done because they don't quite fit within the rest of Keyboard Maestro).
I don't think there is anything magic to this, any good programmer could apply it, and there are others out there who clearly do, resulting in great indie programs like PCalc, Acorn, NetNewsWire, etc.