As a game dev who's been learning ASM recently this was very fun to read through. Wish there was a compiled ROM I could try, but I guess I have to do it myself!
I'm not really into this space but he lists some bin files in the Downloads section. Is this not the compiled ROM you're seeking?
For what it's worth, I was able to simply rename the ET_Fixed_Final.bin file to ET_Fixed_Final.a26 (which might not even be necessary) and drop it into the ROM folder of my Atari 2600 emulator (running via https://knulli.org/ on my Anbernic RG40XXV). It booted up just fine.