It’s not that it’s highly coupled, just that it’s still the early days and only x86_64 was on the roadmap. Arm64 is planned, and will hopefully make it into Ruby 3.2
And with an Arm64 backend, adding RISC-V is probably going to be a walk in the park.
It's not like new architectures appear very quickly, much less adopted very quickly. The benefits of maintenance overhead reduction and development speed increase, far outweight the theoretical downside of having to port LLVM to that new architecture.