Why would pixels perform worse just because other pixels have been cropped out by a cropped mode?A cropped sensor will always perform worse than a larger one. You can't cheat physics.
Or perhaps that's not what you're saying. But jedy was talking about a ff sensor in a crop mode, not about a physicially smaller sensor versus a larger one. When you operate a FF sensor in a cropped mode, the ONLY penalty in performance should be that large parts of the image go away. The pixels that remain shouldn't be any noisier than they were before.
I'd certainly expect an APS-C sensor of (say) 32MP to do worse than a full size sensor of 32MP (all else being the same), but that's not what he was talking about; the appropriate comparison for what jedy was talking about would be an FF 83 MP running in some sort of crop mode and giving 32MP images, versus an APS-C 32MP sensor. Those two should have identical performance, provided the same tech was used to create both sensors.