After a collision, the periapsis is at most as high as the point of collision, and the apoapsis is at least as high as the point of collision. In plain English, if two satellites collide with each other at an altitude of 450km, it doesn't matter what the original orbits were; 100% of the debris will have a low point in their orbit at or below 450km, and 100% of the debris will have a high point in their orbit at or above 450km.
One of the quirks of debris deorbiting is that drag is exponentially higher the lower you are in orbit, and any drag that the debris experiences at any point along its orbit will manifest in a reduction of altitude 180 degrees along is orbit on the other side. So if you have a piece of debris with an eccentric orbit, let's say 300km at periapsis and 1,000km at apoapsis, after a fairly brief amount of time you'd expect the debris to have a periapsis of 299.9km and an apoapsis of 500km. Then perhaps 299km periapsis and a 350km apoapsis. etc. I'm making these numbers up but you get the idea: the high point in an orbit is the part that drops the quickest.
Even if you were to launch 100,000,000 full sized satellites into orbit at 400km altitude and deliberately orchestrated Kessler syndrome, space would be unusable for a few years, but would be 100% back to normal after 5 years.
The dangerous orbits are those in the 500-2,000km range. Satellites whose orbits never bring them low enough to experience significant atmospheric drag. Those are the satellites this new rule is targeting.