Well, that's just false. Everyone can understand pointers given both time and interest. I realize Joel likes to think him and people like him are "special" and born with innate super powers, but we have overwhelming evidence that it isn't true.
I certainly wouldn't recommend hiring someone to write embedded systems that don't understand pointers, but if you are hiring a generalist, a good engineer can figure it out if it becomes a job requirement down the road.
These are the reasons literally everything Joel says should be taken with the biggest grain of salt you can find. He seems like a smart dude, but like a lot of smart dudes that are aware of their intelligence he highly overestimates (and overvalues) his particular opinions.
My advice: don't be like Joel and use anecdata to drive your theories on what constitutes a good interview. You're already writing exactly like he does, and that's not beneficial to you or to those you're interviewing.