TL431 –parasitic capacitor
You could try, indeed, to extract the parasitic capacitance from the frequency response you've shown. For instance, if you take a resistance \$R_L\$ close to the one in the controller, assume 10 kohms, then you read the graph from the -3-dB point and infer a cutoff frequency of 3.5 kHz which would roughly imply a capacitance of \$\approx \frac{1}{2\pi\cdot10k\cdot3.5k}=4.5\;nF\$
However, it is rare that the pull-up resistance used in the data-sheet matches what you'll have on the board. Actually, what matters is the equivalent resistance &quot;seen&quot; from the feedback pin and, very often, there is an additional internal divider in the chip. The best is thus to extract this capacitance as excerpted below from my APEC 2018 seminar:

You can use a network analyzer for doing so or a simple oscilloscope will do well as I explained in the seminar. The thing is to recreate the operating conditions of the opto with the selected controller. A dc source adjusts the bias to position the collector voltage at the same nominal level found in an operating board and you inject an ac modulation via the LED. The response observed on the collector will reduce in amplitude as frequency increases and you can determine the 3-dB cutoff frequency. When you have it, you can apply the above formula.
The coupling capacitance you mention, as its name implies, characterizes the parasitic capacitance between the primary and the secondary sides brought by the optocoupler.