Erik,

think of the port-pin as a junction of the internal and an external driver.
Logic-0 wins, hence the reference to'wired-and'.
by setting a port pin to logic high you don't set it to input, because the MCS-51 doesn't have any port direction registers - however it can but must not be driven low by external hardware.

> but the remaining ports are not exactly low Z in their high state either

sure, they have a weak pull-up in their steady state, and thats the reason why they are in fact bidirectional.

Cheers, Roger