BJT analysis resistance at the base using voltage divider
"....both of the ends of the divider are connected to 0 Ohms with respect to ground and as far as impedance (or resistance) is concerned the two are in parallel."

This sentence is uncomplete. It depends from where you are looking into the circuit. As seen from the supply voltage Vcc, both resistors are, of course, in series. However, the input resistance of the whole circuit - as seen from the signal source (to be connected to the base node) - is most important. And referenced to this node, the signal sourse sees two resistors which are (ac wise) connected to ground: 80k directly and 20k via the zero internal source resistance of Vcc. That means - as far as the input resistance of the amplifier stage is concerned, both resistors are to be considered in parallel.

In general, if you have two resistors with one common node, you cannot say if they act in series or in parallel - it always depends from where you are looking into the circuit.