playdemo and ply are just normal commands to play/watch a demo
The command
timedemo DEMONAME is what you want.
playdemo and timedemo normaly use the Nexuiz/data directory.. ply is an alias that does the same as playdemo but uses the Nexuiz/data/demos directory. I use this alias
alias tdem "timedemo demos/$1"
to have something easy for timedemos..