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.