Read the link white paper. The triangle you point to is discussed, thus:
"What on earth does this have to do with ProPhoto? Well, I'm glad you asked. It has to do with the gamuts of current output devices. They're not that big. Certainly, in volume terms, they are ALL pretty much smaller than even AdobeRGB. This includes - Lightjets, Lamdas (see Figure 5), Thetas, Pegasusses (Pegasi?), Canon, HP and Epson inkjets (see Figure 4), Dye Subs, and You-Name-Its. Not one of them has a gamut larger than AdobeRGB in total. However, some (many) do have gamuts that do not completely overlap AdobeRGB. In some case, as much as 5 to 10% of their gamut may be out of Adobe RGB. Typically, it is saturated yellows that are the culprits (sometimes very saturated cyans and magentas, too, although this is less common)."
"So, unless you are shooting canaries (or more realistically, very unusually intense sunsets - see Figure 6), almost all of the tones you can print or are likely to be able to print in the short to mid term future, are nicely contained within AdobeRGB. So working in Adobe RGB rather than ProPhoto RGB makes more sense (you'll see why in a moment). Of course, for those images that DO contain out of Adobe RGB gamut colours that ARE printable, you probably would choose ProPhoto. But you'd be doing so for a sensible, distinct reason."