That notice is not worded in the best way and hasn't been updated for a while. However, progress has been made.
On the Recoil engine releases (https://github.com/beyond-all-reason/RecoilEngine/releases) page, we have had experimental Linux arm64 builds since March of this year.
Additionally, several people are trying to get the graphical pipeline and overall build working on Mac; you can follow their progress at https://github.com/beyond-all-reason/RecoilEngine/issues/936
The challenge with arm64 is that Recoil Engine does deterministic lockstep simulation using actual native floats for performance. Porting all of those operations (which were implemented using the mentioned library) to work on arm64 and produce the exact same bit-perfect results as on x86_64 has been challenging.
It's great to hear, yeah I researched this topic a bit further and there are clearly some very determined people who want to bring BAR to more platforms, excited about the progress! Floats will be a big pain in the ass for sure and will cause countless desyncs, same story prevented playing supreme commander FAF on ARM processors, though a lot of attempts have been made