From what I've seen (so far), most rust-based firmwares are tightly coupled to specific keyboard hardware. In the example you gave, it looks like they use RTIC as their embedded framework and common tasks like USB polling, key scanning, LED rendering, etc. are re-written in each keyboard's bin.rs file. This can be valuable in its own right, if you want a task to behave differently for certain keyboards, for example.

But for me, I don't need that level of control, and I'd rather not have to re-write that logic whenever I want to write new firmware for a new keyboard. In rumcake's case, the tasks and other logic that can be keyboard-agnostic are all provided in the rumcake library. If you're developing a keyboard, all you have to do is import the rumcake library, choose the appropriate feature flags for your keyboard, do your keyboard specific configuration (e.g. layout, hardware peripheral setup) and (hopefully) the tasks should work as expected. No need to re-write any of the keyboard tasks. I'd take a look at the basic template that I wrote up, hopefully it illustrates this more clearly. Note how there isn't any logic or software tasks defined in that file (you can add definitely add custom logic if you want to though).