The short answer... nonsense. If he is taking X-Plane/Planemaker "out of the box, as is" - which it might seem he is - there are limitations, simplifications and bugs, this is true. But it is in no way "bad". However, serious design teams "bend" the flight model in some areas to fit their aircraft using plugins, so in theory you can do almost anything by reverse engineering and programming like we do on the 737. So it is in no way impossible, but you off course need to know what you are doing. About "fake pitching" moment and CG, I'm not sure I understand what he is talking about, but X-Plane uses Radii of Gyration for momentum calculations. He might have missed that. It is just not practical or realistic for any normal XP aircraft designer to know how much each aircraft part weighs, it's distribution/density etc. Only the manufacturer of the aircraft will know that, and all aircraft are different.
Should he have documentation for what he believes is off, the great thing about XP is that we can always discuss this with Austin (we do it often) and have it corrected fairly soon if we can prove it. X-Plane' flight model has been in development for some 20+ years, during this time hundreds of engineers have been working on improving it, and we will continue to do so