Well for starters it was just a rumor. Secondly lol if you don’t believe devs will be making games cross gen for years as is the case every gen.
I suppose you can chalk it up to just being a rumor, but it was put forth by someone who has contacts in the gaming industry.
And yea, there will be cross-gen games. And they are the ones that don't look all that impressive when compared to games made with the new HW exclusively in mind. You will be getting very similar geometry, the game running at a higher resolution/framerate, with maybe a few bells and whistles thrown in. It will not be a night and day difference.
You ignore exceptions I think.
Battlefield 4 and CoD:AW had significant disparities visually between gens. Textures, geometry, effects, etc. These were impressive looking games at the time, period.
Rise of the Tomb Raider and Forza Horizon 2 had very different visuals between gens. They weren't using the same assets. Geometry was very different as well.
Witcher 2 on 360 lowered the geometry and pretty much everything else.
A more recent example is Dragon Quest on Switch. Much like Witcher 2 on 360, geometry was lowered along with many other effects and assets.
Its worth noting sometimes games have had lower quality assets and effects for a reason, better performance and resolution.
MGSV for example did have assets very similar to 7th gen, but maintained a high resolution with 60 fps.
CoD keeps boosting visually quality while compromising performance and resolution. Its barely a 60 fps game unless you have X1X and Pro.
BF games have great resolution, performance and assets, but the pop in is jarring.
There are many examples I could make but I think thats enough.