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...?
Yes, it is odd at best. I just read an article on the step pyramid, and found no reference to grain silos. I did discover that its estimated construction time is between ten and fifteen years. That rules it out as a grain storage facility in Joseph’s time—as did all the other details.
I can’t imagine what Carson was thinking of, to say something like this. I guess he’s not expecting it to get much coverage—and it likely won’t.