Is Plex Media Player based on Kodi/XBMC?

No.

Our previous-generation home theater app (Plex Home Theater) was based on XBMC's "Frodo" release version. Plex Media Player, however, is not based on Kodi (XBMC) in any way and shares no code with that project.

While Plex Media Player is no longer based on it, we do want to thank all of the people behind Kodi, since Plex could never have started without it.