What you see in that image is only a visualization, not a literal display of all real chromaticities.

A monitor can only show colors within its own gamut, so any chromaticity diagram viewed on-screen must be converted into the monitor’s RGB space. Colors outside that space cannot be displayed exactly; they are approximated, clipped, shifted, blurred, or otherwise adjusted to give a useful visual guide.

So the blue you see outside the sRGB triangle is not proof that your monitor is displaying that actual out-of-gamut wavelength. It is just a screen-rendered representation intended to show where those colors lie relative to the gamut boundary.

Also, some published diagrams are not strictly rendered in sRGB and may use altered processing to reduce harsh discontinuities, so the displayed colors can be even less literal than they appear.

In short: the diagram is conceptually correct, but the colors on your screen are approximate. There is no “real” on-screen chromaticity diagram that can faithfully show all visible colors on an sRGB or limited-gamut monitor.