All the custom ROMs out there that are not Touchwiz-based don't use the stock camera (and the stock camera can't be installed on them). So, by using ROMs such as Lineage OS, Resurrection Remix OS, crDroid, AOSP and so on, you will get lower quality than on Touchwiz-based ROMs such as SuperMan-ROM, BatMan-ROM, IronMan-ROM.
As a programmer, I'm not sure what that means but I'm inclined to think there may be some truth to it or he knows what he's talking about.
When cgirerd says "stock camera" is he referring to the stock camera application, or the stock camera firmware/drivers for the ROM? I have an Samsung S5, and I use LineagOS and I've never noticed even a slight degradation in camera quality; but, I'm also of the mindset that the world we be a better place if phone didn't have cameras and almost never use my own.
Is there any truth to that post by cgirerd?