One of the big issues with Go is that it's got a steep learning curve. (There was an article about this posted on HN yesterday, in fact)
Rust, by contrast, is an extremely easy language to learn. Of all the languages I've picked up at least partially in my ~12 years of professional programming, Rust was probably second only to to PHP in "get up and start running with it".
If Discord is trying to pick up new developers, or cross discipline seniors to improve their talent pool, Rust is definitely a better choice than Go.