About how long would it have taken to build the step pyramid? Is this known? Joseph had seven years, starting immediately [i.e.: as soon as he was put on the job] to store the grain. Where did he keep it while the pyramid was under construction?
A pyramid structure is not even remotely ideal for use as a granary. The timing is wrong (regardless of your preferred chronology, I'm not aware of anyone who believes Joseph was connected to the pyramids). There's no sign of grain storage besides maybe a few pots (ostensibly a food stash for the dead mummy's travels in the afterlife).
Carson's not stupid, so I have to wonder what his thinking is here and where he got his ideas. I suppose everyone has pet ideas they may nurture without much investigation/thought, but to present such in a public speech is absurd, and in the context of a presidential campaign...?