You can't use the Linux kernel implementation as soon as you want to be cross platform and the only official userspace implementation is in Go which is awkward to FFI into from Rust.

I agree with your point. Use what is out there. We did that by using Cloudflare's boringtun for a long time. But it has limitations that eventually we could no longer work around if we wanted good compile-time safety and performance.

I can use sanstun in no_std environments like on an ESP32.

Can I do that with Jason's reference implementation?

Let people build. They might have reasons you haven't foreseen.