Don't know that I want to wade into this debate....but like mounts
, optics can indeed vary in quality, and if a system is "diffraction limited" who cares if it is any better?
"Diffraction limited" can mean a lot of things, I suspect it is a variation of the Raleigh criteria...ie at this point of optical correction you'll be able to split a double star of separation "x" arc seconds (x being a function of aperture only) but for manufacturers to suggest at this level of correction in optics the image can only be bettered by increasing a telescope's aperture is, well, utter rubbish!
Nature again is rarely so kind!
Tube currents, local seeing & the earth's atmosphere collectively bugger up a telescope's image.
If the quality of that image is already hovering on barely adequate, then all of the above will make sure it is less than that.
If however the optics are near enough to "perfect", the image can be disturbed 20% or so before you know it's not looking too good....
Cheers
Peter