Incorrect. Wide loading screens are supported on 3.3.5a. Go to LoadingScreens.dbc and set last field to 1. Note that wide version of your loading screen needs to have correct name. For example:
In DBC is:
3, Kalimdor, Interface\Glues\LoadingScreens\LoadScreenKalimdor.blp, 1
In actual MPQs are 2 BLPs:
Interface\Glues\LoadingScreens\LoadScreenKalimdor.blp
Interface\Glues\LoadingScreens\LoadScreenKalimdorWide.blp
So just make sure that naming in correct, in case it was changed later (afaik it was not).
Editing wiki, correcting mistake which was there.