Wow...that's a pretty crazy approach. Userland x86 emulation on an arm device, and a ton of ASM code and a sort of JIT. Guess options are limited in a walled garden.
x86 is indeed a weird choice for an "emulated" architecture - I'm pretty sure that RISC-V would behave a lot better (due to being a legacy-free design, if nothing else), and it does have support for running Linux.
I just went down a rabbit hole on the subreddit for the iSH community, and it looks like the creator went with x86 because it's the only architecture they knew at the time.
Edit: Found an HN discussion about it: https://news.ycombinator.com/item?id=18421016