How can an image properly illustrate (in RGB) what colors are outside of the RGB gamut? That image must be inaccurate? Should I assume the image is technically incorrect but a good illustration of what the RGB gamut does look like?
This (and every other gamut picture in existence) can't, which is why the colors don't change any further outside the RGB triangle in these illustrations online. (In CMYK print it's even worse.) You have to use your imagination that the colors become even more intense.
A good parallel would be if you ever try to take a snapshot of a vivid sunset on your phone, then compare the screen image with the sunset behind you. The sunset's colors will be far more vivid, saturated, intense, than anything your phone can display. Those are some of the colors outside the RGB gamut.
The closest comparison would be to try to match the colours of the sunset with a paint app on your phone.
The phone's camera has its own limitations which prevent it from reproducing certain colourful settings well, even if you viewed the photo on a wide gamut screen.
From the article: "All of the diagrams in this blog post are in sRGB color space. That means that all colors outside of the sRGB gamut aren’t accurately reproduced in the diagrams in this post!"
Your computer monitor works within the sRGB (or comparable) color space, so it physically cannot render colors outside of that color space. When you look at that diagram you're seeing a representation, not the real deal.
Pigments could theoretically do a better job than pixels. But those would have to be some very good pigment$. I'm not sure if there is a printer that can produce this in practice. Your cheap inkjet printer definitely won't. The most expensive printers haven't been designed to cover the full human visual spectrum either, out of technical limitations, or cost limitations. And even when you have the pigments on print, the light $ource would have to contain the entire visible spectrum at high intensity (basically you'd need a very bright white light, as bright as the sun). And then the surface cannot have any glare. etc. So not very practical to achieve.
