By default, Plex only encodes with this unraid build and the GPU passed through. There's a way to enable decoding as well; but it is not officially supported by the Linuxserver.io, Plex, or unraid teams.
Here's a script I've modified that can be used via the user scripts plugin on a schedule to enable the decoding support:
https://gist.github.com/Xaero252/9f81593e4a5e6825c045686d685e2428
I will update that script as needed, until Plex adds official nvdec support.
Also note that nvenc and nvdec DO NOT offload audio transcoding. This generally is of no concern, unless you are dealing with lossless Atmos audio, which can be quite substantial to downmix and transcode.