Zero Sum nature of Commodity derivatives/Managed Future

The module explains it as if you are long commodity future, and you invest similar amount in the treasury bill of the same period as future, essentially its a zero sum ####… can someone explain this to me … Also seond question is CFAI text gives example of CTA(commodity trader adviser) in the managed future… is that so, i thought CTA’s are separate class of alternative assets and different from managed future… any clarity is welcome!!!