I would not be too sure about that.
I just played around with that Reaper build on my DTK running Mac OS 11.0.1. Reaper is able to load AU and VST plugins compiled for x86_64 only, with AU being silently bridged by some layer obviously supplied by Apple (Logic uses the same) and VST being a bit more obviously bridged by Reapers own implementation that was already available to bridge e.g. 32 Bit x86 plugins running in a 64 Bit x86_64 environment. You can see that AU bridge in the activity monitor, look for
AUHostingCompatibilityService (Reaper) there.
I also managed to run a universal binary VST3 natively in Reaper, but I’m not sure if in case Reaper detects that the ARM part of a UB cannot be launched, the bridge is being used silently in the background?