I suspect that Skittles is still around and that he'll be important to the story again. But wouldn't that make for a sad, funny story. Skittles wakes up one morning feeling lucid and sane. He remembers the garden, so he jots down the location on a scrap of paper. Then he heads off to join his friends at the Tower only to realize, too late, that he's no longer invisible to the zombies!
An alternate theory. The zombie with the address could have been from Fort Irwin. Col Kimmet did say that there were "special ops" going on, and there was reference to a "squad going down". Maybe the zombie with the note was part of a recon of LA. Maybe there are survivors still there.