I'm not the author. I'm just fascinated with size optimisations and I thought that it was an interesting achievement. But yes, the simplest optimisation is to not write code that you don't need to.

I've had my fair share of x86 assembly programming, and I used to do all sorts of tricks back then as well (I vaguely remember calling DOS interrupts just to get multiple registers into the state I wanted them when speed didn't matter as much as size), but I don't think that I'd have been able to cramp a working chess program in such a small amount of bytes.