I wonder why they chose to use the tiny watch-size amoled board for this. Wouldn't a multi effect UX be better with one of the 4-7" touchscreen boards?

Also: Am i reading it right that you can get an ESP32 with a 7" touch screen for like $30?

It's the author here. AMOLED is beautiful to look at. And you can stick this device on your guitar. Having said that, the same code can be compiled to any ESP32-S3, even headless boards, to use in a custom pedal (like the ones with Daisy Seed 3). CoyoPedal uses Gea Stack that allows us to target any device, including macOS and even the web, https://coyopedal.playtaurus.com has the exact same DSP and UI code built for WebAssembly.

The larger screens like the CYD are quite awful. Blue tinge, bad viewing angles, low brightness and slow touch response.