"Polyphasic capability" here refers to a system of napping that (in theory) greatly reduces the number of hours you need to sleep. The mailing list has been debating to what extent polyphase could have at one point been an evolved ability that perhaps we lost for some reason. In this particular thread, they're discussing survival-selection versus sexual-selection:

"In the case of human beings, there may be a sexual-selection aspect to
polyphasic capability, but the salacious aspects of an exploration of that
possibility need not distract us here."