Why do people still believe that? It is wrong. Here is a great thread that explains in detail what is new and what isn’t: MSFS - High Level Overview of MSFS2020 Tech | FSDeveloper
The graphics engine may be built from scratch, but that is one of the things most people seem to be extremely happy with, me included. Some of the terrain stuff is entirely new, and while it is mainly great it has some very serious teething troubles, terrain spikes being the most glaringly obvious one. Sound is all new, and is absolutely amazing and in my opinion the biggest success of this sim.
But huge parts of the sim remain from FSX and beyond. Most scenery stuff (other than terrain), most offline AI traffic stuff, the flight model is a new way of computing behavior based on mostly legacy format data. Furthermore, the legacy code was available for Asobo to build upon.
Reading threads like this one (and man have there been plenty!) it seems one of the biggest disconnects is between people thinking “the sim is all new, have some patience, in a couple years they will have it right” and those thinking “this is just one more generation of the MSFS franchise, why is quality so much lower than the previous three or four generations of MSFS”.