I don’t know who says that beside people who must not like bare-metal servers 
I’d say go Server so you can understand how to set stuff up proper, and run into server-specific issues with traditional troubleshooting methods vs something Silverblue/Immutable/container-related. I’m not totally sold on containers or immutable.
I’ve ran Plex on Fedora Server and other Linux distros bare-metal no problem! If I recall right I installed an RPM they provided directly, enabled the service, and was good to go for just Plex on the network (video files and permissions where a whole different jungle that had me just going Kodi and a simple vsftpd set-up :p)