The article doesn't make any effort to explain the orbital mechanics and ultimate dynamics of a Kessler Syndrome.
The assumption I think you're making is all debris remains in a fairly localized decaying orbit, which is not the case when a high energy collision occurs. The debris will fan out in two radial smearing patterns that loop back around at the collision point. Some pieces will hit more atmo for longer periods while others will achieve higher altitudes for a time and those can debris fields last longer. Should a LEO cascading collision event happen we would see lots of debris fields reaching higher orbits and a non-zero risk of those satellites being hit too.
Sentinel-6, GRACE-FO, ICESat-2 are among only a few of the many numerous LEO observational satellites that are at risk should a Kessler syndrome occur... our ability to watch and build predictive models of what's going on with our ecology can/will be lost; this is a huge added problem to the ongoing global ecological collapses and affects our ability to mitigate climate change
Not to mention all the communications systems we have in orbit that we rely on for our economy which would result in significant loss of logistics / economic efficiencies and straining growing social political issues. All of which means higher likelihood of damage to local and regional ecologies
The assumption I think you're making is all debris remains in a fairly localized decaying orbit, which is not the case when a high energy collision occurs. The debris will fan out in two radial smearing patterns that loop back around at the collision point. Some pieces will hit more atmo for longer periods while others will achieve higher altitudes for a time and those can debris fields last longer. Should a LEO cascading collision event happen we would see lots of debris fields reaching higher orbits and a non-zero risk of those satellites being hit too.