Why on Key agreement the check when all z xores are 0 not performed

According to [documentation](https://github.com/equalitie/np1sec/blob/master/doc/protocol.pdf) on group key agreement the following check seems to be ommited: z1⊕ . . . ⊕zn = 0 whilst mentioned on original Pointcheval's GKE+P protocol.

So I wondered why is ommited? I also asked upon this as well: https://crypto.stackexchange.com/q/66544/59291

