Isn't the traditional advice that if you try to write both a game engine and a game that you'll get neither?

Braid and The Witness, his last two games, both used custom engines written in C++.

Then it's a good thing not everyone listens to traditional advice.

I mean, it's not like there are no games in existence that shipped with a custom engine.

Even in hindsight it's hard to judge whether building your own engine was good or bad decision, and we are nowhere near "the hindsight" level of knowledge.

Yeah, and it's taking him YEARS to implement a simple grid-walking Sokoban clone, and Braid anniversary edition had to be delayed.

Last I heard, the Sokoban game has a ridiculous number of puzzles on it. Can't find a source but I seem to recall hearing that it would take 400+ hours to finish it all. So.. I don't think it's entirely unreasonable it's been taking this long.