I don't think I made it too complicated. The image you show has everything all together on one page. I showed everything in linear fashion, one step at a time.
If you want my solution to look less complicated here's the version without all the steps shown in excruciating detail:
As to your question, the large quote you show apparently was posted on another forum I haven't seen, and I believe there are some errors in it.
Look at the Bode plot associated with the original problem statement. It shows an upper corner frequency at (Gm1 Gm2 Rout2)/CL due to the pole in the expression for Zin. It is not exactly correct, but it is a good approximation if you can neglect the (+1) term in the denominator of the Zin expression.
The denominator of the Zin expression is (Rout1 CL s + Gm1 Rout1 Gm2 Rout2 + 1). If this is solved for s, the result is:
(1+Gm1 Rout1 Gm2 Rout2)/(Rout1 CL) which is the exactly correct pole frequency.
In order to get the pole frequency shown in the Bode plot we have to neglect the (+1) term
and solve (Rout1 CL s + Gm1 Rout1 Gm2 Rout2) for s. Then we get a pole frequency of (Gm1 Gm2 Rout2)/CL
The large comment says "If you inject a current into node X...This gives you a low impedance at low frequencies. How low? Just 1/(gm1rout1gm2)"
That is not the result I get for the resistance at node X. To get the resistance at node X, simply remove the +s CL term from the admittance matrix and invert it. The resistance seen at node X is the red element of the inverse:
In order to get the expression in the large comment (from another forum) you show, it's necessary to ignore the (1+) term in the resistance expression I show in red. I believe the expression in the large comment is in error.
The expression in the large comment for the resistance seen by CL suffers from the same neglect of the (1+) term. The commenter simply asserts: "...you can disconnect node X from all but the gm stages and realize that CL sees a resistance of 1/(gm2Rout2gm1) and so the pole frequency is 1/(gm2Rout2*gm1)"
He doesn't show how he calculated that "CL sees a resistance of 1/(gm2Rout2gm1)", and then he makes a mistake when he says "the pole frequency is 1/(gm2Rout2*gm1)"; the pole frequency would be gm2 Rout2 gm1/CL if the expression for the resistance seen by CL were correct. Perhaps he didn't actually calculate the resistance seen by CL, but just asserted that it is 1/(gm2Rout2gm1) because that would match up with what is shown on the Bode plot.