TINA-TI: Opamp Stability analysis Video-5 (Exercise) (Precision Opamp Labs) (Riso calculation)

The example you refer to talks about an op amp in gain of 20 - this is equivalent of 26dB and NOT 20dB.  Thus, in order to find the frequency of interest, you may either find the crossing of AOL with 1/beta (26dB line) or look for zero crossing of the loop-gain, AolB - in each case you should get the same answer, which seems to be around 15kHZ - see below.

The whole idea of using 20dB gain line and loaded_AOL to determine Riso value, for stabilizing the circuit in a buffer configuration, has to do with the fact that you need to have a decade of frequency range left to recover its phase margin to an acceptable level (>45degrees) from the pole-induced phase dip before AOL_loaded zero-crossing (where loop-gain is zero).

But actually, it is equally important to introduce the cancelling zero within a decade of the frequency where a second pole occurs so the phase margin is not allowed to dip too low before cancelling zero recovers it to the acceptable level. This typically requires Riso resistor to be at least 1/9 of the open-loop output impedance, Ro. Of course, the best case scenario would be to determine the Riso resistor based on the location of the second pole, resulting in a minimal phase dip, but this would require Riso value to be equal to Ro, which may not be acceptable since it will cause a gain error in circuits driving resistive loads, RL – in such cases, using a double-loop stabilization method eliminates this problem.