1) The RGB palette used in monitors most certainly does not span the whole gamut of human vision. Not even the wide-gamut monitors used professionally can do that yet.
2) The diagram showing each cone type's peak spectral sensitivity is a bit misleading, too. The M- and L-cones are colored red and green, but if you notice carefully, their peaks lie in the green/yellow area, so characterizing them as red/green is misleading. And "each of our cones seeing a primary colour (blue, green, or red)" is flat-out wrong. Every cone fires in proportion to its overlap. The author clarifies this later, but it's best to tell people the caveats up front. Continually referring to "primary" colors is adding to the confusion here.
3) WhIle they did cite an article pointing out that ratio-computing cells exist in the retina, it does NOT follow that "it is the main source of information that the brain has to help it identify the wavelengths of light". The paper itself only talks about spectral separation in retinal cells pre-brain. I don't know where the author got the idea that ratios are the main source of info sent to the brain. Amplitude information is sent, as well.