I also wrote keyboard firmware in Rust and thought it was great! I leaned on embedded-hal and the chip HAL crates (one layer of abstraction higher than the author in your link) and was able to get the majority of the firmware done in a day or two. For example, here is my key matrix scanning function:
https://github.com/bschwind/key-ripper/blob/d33db6144bbb6f80...
I also gave a small talk on the subject (aimed more at beginners) at a Rust meetup in Tokyo:
https://www.youtube.com/watch?v=x7LQevYn7d0
And finally, a (somewhat) dated article I wrote about using Rust for embedded work:
https://blog.tonari.no/rust-simple-hardware-project
Overall, it feels totally usable to me, and I love how Rust's ownership and lifetime concepts prevent me from doing stupid things like reusing a pin for two peripherals or configuring a peripheral with pins it can't work with.
People who have used Zig to do embedded work have reported the opposite: they like Zig because it helps keep simple things simple and not more unsafe than they have to be.
https://kevinlynagh.com/rust-zig/