Problems with deriving output resistance without using a model
How they obtain Rsource/β conceptually.


let Rsource=Rb


if Rb is biased from some voltage Vcc  
such that the value of Rb controls Ie  using KVL;


Ib=(Vcc-Vbe-Ve)/Rb 
thus Ie/β =(Vcc-Vbe-Ve)/Rb
or (Vcc-Vbe-Ve)/Ie = Rb/β
let Vcc and Vbe be constants and Zout=Ve/Ie

the change in emitter voltage from an external current can be nulled by an opposing base current amplified by Rb/β thus Zout = Rb/β // Re 


q.e.d.

in conclusion the Emitter Follower
 Zin = β Re and ( for any Ze )
Zout = Rb/ β   ( or any complex Zb parts)

For future reference


a transistor amp with negative feedback using Rf from collector to base with Rseries to a source voltage input,  
we get voltage gain Av= -Rf/Rb  and it turns out that the input and output impedance becomes a function of negative feedback gain such that Zin is reduced by feedback ratio and Zout at collector is reduced by feedback R ratio //Rc


The same is true for Op Amps but the gain is so high we consider the differential input impedance to be zero and likewise for output impedance until current limiting saturates the driver.