Is Michelin specifically referring to karting tires here? I ask because in a car, obviously a wider track width is going to allow for better cornering ability in most cases, but in a kart it’s a little more nuanced, since the inside rear wheel has to unload for the kart to rotate effectively. To do that, you need to get the kart to flex and tip onto the outside rear tire. With a wider track width, you effectively are lowering the relative center of gravity, and diminishing the kart’s ability to lift the inside rear through weight jacking. You’re also relatively softening the rear of the kart, as you’re exposing more axle and allowing more flex. I find that wider rear track widths offer more stability, as the kart isn’t as narrow and reactive to steering inputs, so that’s how I use rear track width when fine tuning the balance.
This comes back to the constant struggle to separate “grip” from “rate of lift”. A kart that is too narrow will be reactive and unstable, and some drivers may perceive this as “lack of grip”, when in reality, they have the opposite problem. The kart is unstable because it is digging too hard on the outside tire from the excessive weight jacking. Conversely, a kart that is too wide can start to get too soft to the point where the rear end just squats in the corner and the kart flat slides or drags the inside rear wheel.