Few quick theories; test at your own pace.
I poked around in Garry's code by looking in gmod/gamemodes/sandbox .. player.lua and command.lua. (He REALLY needs to update code.garrysmod.com - it's quite out of date.
First;
Isn't prop_blah_blah an entity class, not a model? The references in his code all seem like it would be a model.
(Still seems that should return false though)
Second;
It's been my experience in Lua to place function BEFORE the call to the function. You currently have it backwards.
No idea if that's required for it to work, but it's definitely good practice.
Third;
Tried making functions local? See UNoLimited as an example. (I'm not 100% sure why Megiddo made them local, but I saw that it was, kept it that way, and it worked)
Also, part of this theory...anytime you attempt to spawn a prop, Gmod tries to check to see if the sbox_ limits are set for it... it's possible that function is getting called before yours is.
Fourth...
You're forcing this to run on the server and not the client, right?
Someone else could help here I'm sure.
I'm gonna go play some Fallout 3 now. Had no idea a "choose your own adventure" book (in theory) had been turned into a game.