Finding this tutorial was how I discovered Interlopers.net. I was trying to customize my HL2DM menu and thought I was doing it wrong. But I dl'd a few supposedly working menus, and those didn't work either, so I knew that Valve had changed things around.
Poking around a bit, it seems that this is part of the new loading scheme with Steampipe. The game defaults to loading everything from the .vpk files. But then, it lastly checks your "custom" folders, for instance:
~ Steam/steamapps/common/Half-Life 2 Deathmatch/hl2mp/custom/(yourname)/resource/
Supposedly, Valve won't copy over anything the users puts there, which is nice. IDK if the custom folder is old news for people, but I searched a lot of menu topics this week and didn't see it mentioned anywhere.