What characteristics of the Onkyo would account for any of those alleged (and false) claims? The answer is none, because none of those limitations exist.
The people who insist on upgrading and/or tweaking CD players are coming from an analog background. They are used to turntables, where small changes do make a big difference, and where optimum performance costs a lot to obtain. Digital is an entirely different paradigm, and some people simply cannot get their heads around the fact that perfect sound is an inherent property of the format itself, hence shared by all players.