Only if you assume all players only ever use futures. But make an interest synthetic contract (short future long underlying) and you're out of the zero sum regime again. And it's enough that one actor is not inside the zero sum regime to make that apply to the whole game.
Again, it's a great approximation most of time and over most time periods and asset classes, but it is NOT axiomatic in the way most people believe it is.
Remember: as long as there is a way to inject or withdraw more capital into the system (through whatever asset class, as they are all interconnected), the sum is not identically zero.
Just assume one of the stocks is a gold mining company that works efficiently. The share value rises, and the shares are redeemable for the gold, without anyone having to lose anything (except mother earth)