The Emperor has no clothes.
Are they simply saying RGB can't emulate 'redder than red' or 'bluer than blue', only the colours between the red and blue that are used as the source? That idea has merit.
Just because you don't understand something doesn't mean nobody does. Don't be one of those armchair physicists with no training insisting that relativity doesn't make any sense. The Earth is not flat, even if it appears that way when you look out your window.
People have been researching the topic for many decades, it's pretty silly to just wave your hands and dismiss it because it's technical and you don't want to go through the effort of understanding it.
The reddest red your monitor can produce is not pure red and likewise for blue and green. Pure colors of a single wavelength are more saturated. Lighting up the red elements on your monitor won't produce pure red, and there's no amount of adding of green and blue that will change that.
The same is true for non-primary colors, and would be true even if your red, green and blue elements were completely pure, monochromatic colors. If you combine red and blue, you get a shade of violet, but it won't be as saturated as a real pure violet. You can prove this to yourself. Take a look at a rainbow produced when the sun shines through a prism or a glass ornament hanging in front of a window into an unlit room. In particular, look at the violet end of the spectrum. Now, try to produce that color in a photoshop color picker. You won't succeed.
I'm surprised it's that hard to believe that your monitor can't produce certain colors. It's obvious that it can't produce certain brightness levels that we see all the time. It won't produce pure black and it won't get as bright as the sun, or even as bright as an ordinary light. Why wouldn't it be similarly limited in color reproduction?
If you want to understand more, read up about
metamerism,
