I know from Brads posts that it takes years to create a good gameengine but I don't understand if it's just about "doing it" or if it's trial and error or however it is you do it.
I don't seem to understand the question but I'll try answering it.
A game engine takes years to develop into something that's actual "workable" and suitable for gameplay regardless of the method, that includes testing to see if the thing works. Ypu need an actual team of developers if you want to create in a "short" amount of time (still a few years though).
The fact that he is able to create one by himself, that's actually workable is an achievement onto its own.
We arn't even talking about the fact that he still has to create art, music, and gameplay assets.