Gamut compression in color calibration aims at ensuring all RGB values are contained within the visible spectrum, hence the whole surface within the horseshoe:
That’s because cameras tend to record a bit of UV and make it pose as blue, plus color profiles and chromatic adaptation may push colors out of visible spectrum. So we cleanup after the CAT. And, as a metric of visible spectrum, the closest we have are Rec2020 and ProPhoto RGB.
If every pixel lies in the visible spectrum, then we can work. If not, slider-pushing on things like saturation will only make it worse, and the gamut-mapping at output becomes a fiesta of randomness.