Shouldn't that be completely opaque to the user? I don't have any idea what programming language most of the software I use is written in.
If it's faster or more stable than tmux, shouldn't that be what's advertised, rather than implementation details? Reading the page, what catches my eye is the webasm plugin system (although it's not immediately obvious to me what I'd use that for).
No necessarely, because app being written in Rust and Go is actually a feature for the user, it means the entire app is a single executable with a decent runtime performance.
I do still question what it means if the coolest thing to say about it is it was written in Rust, whether that's intended for developers or for users
That would have been nice indeed. However many languages use lots of memory and are a bit slow, or are security-dangerous like C. It's nice to know, then, that this app likely uses barely any memory at all, and is as fast & keyboard-responsive as it can be, and harder to hack — Rust. (Assuming it is implemented in a good way :- ))
If it's more secure, that's harder to measure. But that's not even the tagline, just that it's written in Rust.
— that's easy to measure, if it's written in C or not (or some other memory unsafe language)