How to differ output & input resistance of small-signal model BJT?
Because, if you ask me, models like this one could be identified as one whole resistor, whose value would be considered as input and output resistance simultaneously.


Your model has three terminals, B, C, and E. A resistor has only two terminals. You can't reduce a three terminal device to two terminals and consider it "equivalent".

If you are using the BJT in common emitter configuration, then the input resistance is, by definition,

$$R_{in}=\frac{{\rm d}V_{be}}{{\rm d}{I_b}}$$

And the output resistance is, by definiton

$$R_{out}=\frac{{\rm d}V_{ce}}{{\rm d}{I_c}}.$$

You can work out what these values are from the model you showed.

If these two derivatives don't have the same value (and they don't), then you can't say the input resistance is the same as the output resistance.