hi,

it seems you do not understand basis clean.
Let explain with next way. Imagine an IC with open drain output (7407). Now imagine that this output is connected to the input of the second IC. Now here are the questions:
1) how make the output of 7407 with high level? It is open-drain! And so to ensure high level we need to use pull-up resistor tied to the output, right?
2) how make correct input of second IC? Indeed, it able to recognize the low level input. But to ensure that it recognizes high level input we need to put pull-up resistor to the input, sure?
3) Imagine that 7407 produces low level output. Which level does the second IC recognize then? Right, it sees low level too. Is it possible to do that 7407 produces low level output but the second IC inputs high level? No - as long as they are tied it is not possible. But it is possible to do vice-versa: if output of 7407 is at high-Z state (due it is open-drain) then we may connect input of the second IC to the ground and low level input will be recognized.

Now come to port 0 of 8051. It should not be a secret for you that basicaly the each pin of P0 is made similar: the open drain output connected to loop-back input. The point of this connection is the pin of P0.

Hope you know the answers now.

Regards,
Oleg