One of the key reasons to sign packages is to ensure nothing nefarious is happening by virtue of an as-yet-unknown third party. Using the signing key’s ID rather than a URL hosted within the Plex infrastructure helps ensure that people can detect tampering since the key ID should (ideally) never change.
As apt-key is already in use, why not use it fully?
Ubuntu isn’t the only keyserver available; theirs just tends to be the most responsive. Your key is mirrored across all of them, it appears.
In a future version of apt-key, the keyserver won’t need to be specified any longer, as it will handle HKPS properly without needing to use the adv option.
But if the goal is to not appear Ubuntu-specific, you could reference the full OpenPGP keyserver pool:
I’m aware that’s it a pretty worst-case scenario that your key and package will both be compromised, but that’s pretty much the case that signing is meant to prevent in the first place.
Also, since I have your ear: the instructions say to write to /etc/apt/sources.list.d/plexmediaserver.list. However, the package also provides a configuration file for that location, and that file has the repo’s source disabled/commented by default. This might be a surprising situation for some, as it is not documented on the page that the repo may disable itself upon package installation.
