Joel's guitar playing alone automatically cancels out any shortcomings he may have with the keyboard.
I also thought that - though I wonder why he has problems with it. If he knows the chords of a guitar it should not be too hard for him to know the piano keys. I used to play a bit (really a bit ;D) the keyboard and my father has some guitars which I sometimes tried to play - I always thought it was much harder to play the guitar than the piano. Who knows...maybe he has other notes on the keyboard, just like which sound he has to turn on for the new song or something like this.
If Joel had post-it notes on his keyboard he isn't the first ever to do it. I know that Don Letts (he's the bloke on the Howling Bells JD Set show aired last May) had colour-coded keyboards when he played with Big Audio Dynamite, because he'd never learned music.
My brother told me this because he knew/knows Mick Jones, formerly of The Clash, who formed BAD after The Clash folded. My brother worked for The Clash for a short time and also worked for a few other groups in the eighties. He got me on the guest list a few times!