Yes, even the so-called "bible" is unhelpful on this point!

The confusion arises because the 8051's "Quasi-bidirectional" IO is quite unlike many (most? all?) other processors' IOs.

Many (most? all?) other processors have a specific register (or registers) that must be specifically configured to make an IO either an Output or an Input. There is no such configuration register on a standard 8051.

With such IOs, writing to an IO configured as an Input is an error, and reading from an IO configured as an Output is an error. But this is NOT the case for a standard 8051.

With an 8051, you can write a '1' to an IO, and then call it an "input" - but there is absolutely nothing whatsoever to stop you writing a '0' to that "Input" and it will instantly force the zero state. This cannot happen on the other processors where the IO has to be separately reconfigured as an Output before a write has any effect!

This is why I (among others) say that it is unhelpful to think of "configuring" an 8051's IO pins as "inputs" - it is far more helpful to think of them as permanently Quasi-bidirectional, but just having the output driver in the 'Off' state.

To further add to the confusion, some modern 8051s do have IOs that can be configured to be specifically "Input" (only) or "Output" (only) - where the outputs actively drive both the '1' and the '0' state!