I finally managed to find the correct reference.
Apple these days only talk about the use of LoginHooks and LogoutHooks in so far as to say that their use is deprecated and that they instead recommend a user LaunchAgent instead of a LoginHook. Unfortunately not only is there no equivalent mechanism for a LogoutHook there are also important technical differences in how they work. A LoginHook or LogoutHook runs as root and can therefore do things a user LaunchAgent cannot.
Anyway, the original and now somewhat obsolete documentation I was looking for is the Mac OS X 10.6 Server documentation, in it, it states that LoginHooks can be applied to a specific computer or a computer group. By omission it clearly implies that it cannot be applied to a user or a user group, this certainly matches my memory and belief.
Therefore you are 'holding it wrong'. 😉 You will have to use a device group instead.
PS. A individual user LaunchAgent is stored in a users home directory in ~/Library/LaunchAgents a computer wide LaunchAgent would be in /Library/LaunchAgents and would apply to any user logging in on that computer.