Mario Kart World generally looks more saturated than previous games if anything, so it definitely meets that minimum. And the article is about they missed out

Mario Kart World generally looks more saturated than previous games if anything, so it definitely meets that minimum. And the article is about they missed out on the optionality of using the additional gamut, but that additional gamut wouldn't intrinsically look better. It's easy enough to edit a screenshot to show us what could have been, but even in that single screenshot there are things that look worse: like the flames gained saturation but lost the depth the smoke was adding, and some reasonable atmospheric haze vanished. (similarly the game in the side-by-side has some downright awful looking elements, like the over-saturated red crystals that punch a hole through my HDR display...) Given Nintendo's track record for stylization over raw image quality, I'm not sure why this isn't just as likely them intentionally prioritizing SDR quality and taking a modest-but-safe approach to HDR... especially when the built-in screen maxes out at 450 nits.