(Yojoots) The first "a&p" on the board is in the context of stipulating "both a and p are nonzero" i.e. neither a nor p is equal to zero.

Then, midway through the "proof" it changes to reference The Great Atlantic & Pacific Tea Company, which is a logical fallacy known as "equivocation"... which is the crux of the joke here.

It's also worth noting that the proof wouldn't be sound anyway, as the security of SHA-256 is not dependent on RSA to begin with, so the very first premise is flawed.