I seen nothing technically wrong with that post.
Redial, the technically wrong part of that post:
It is simply to gather more light. It's as if looking at a painting in a candlelit room, then turning on the lights and looking at the same painting from the same spot.
In astrophotography increasing the diameter does not increase the brightness. To increase the brightness you need either a shorter focal ratio or longer exposure time, assuming the gain on the camera sensor is the same.
In visual astronomy increasing the aperture does result in a brighter image, sort of. Say you have a six inch diameter telescope and compare it to a twelve inch telescope. At the same magnification the twelve inch scope will have an exit pupil, from the eyepiece, that is twice the diameter of the smaller scope's exit pupil, therefore the image will be four times brighter visually.
However, the reason you would want a larger diameter telescope for astrophotography or visual astronomy also, is the resolution will increase linearly to the diameter. So if the James Webb telescope has twice the diameter of the Hubble it will have twice the resolution. But if the focal ratio is the same the two scope's will have the same brightness.
Hope that helps.