I'm saying that there's a /czero/ as well as a /cstrike/ directory when CZ is installed.
So indeed it seems that you get an upgraded CS installation. I'm not sure what measures have been implemented by Valve to prevent people from simply giving out the updated assets. I suppose it would be possible to have Steam check this kind of stuff and ... correct it, if necessary.
They didn't stop this with the Blue Shift content update. :X
You can still use custom content in Steam by dropping them in the SteamAppsusername@mail.com<game><moddir&g t; directory, and you can get the original content by downloading the dedicated server; it has all the files unpacked. I think (but I'm not sure) that mp_consistency only checks specific files, or puts limits on the files, like the size of the model or something. I know you can still use custom player models in TFC, at least, but I think there is a limit to the bounds of the model, and you can only replace certain specific TFC sprites, etc.