Why is the slope of the op amp graph 20 log (AB) in this application note
That Bode plot is a plot of magnitude and phase of loop gain, plotted in dB and degrees respectively.
Loop gain (Aol x beta) is equal to open loop gain minus the noise gain when both are plotted in dB. If they're both plotted on a linear scale then Loop Gain = open loop gain divided by noise gain where noise gain is equal to (Rf + Rin)/Rin (for a purely resistive feedback network the noise gain plots as a horizontal line).
The difference between open loop gain and closed loop gain (both plotted in dB) is equal to 1 plus the loop gain, 1 + (Aol x beta), where 1 + (beta x Aol) is known as the feedback factor.
The difference between closed loop gain and noise gain is that, for a purely resistive feedback network, noise gain is constant over frequency where as the closed loop gain reduces as frequency increases, slowly at low frequency reducing at a faster rate at higher frequency after the closed loop gain plot meets the open loop gain plot.