1) The link I posted states that it is a fork.
2) The packager defined the conflict.
Perhaps someone else with more knowledge about the projects will comment. You can also post in the AUR or directly to the maintainer of the plex package. Welcome to Arch BTW!
]]>Plex is a fork of xbmc so as far as your system is concerned they are providing some of the same files and cannot coexist.
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