To some degree that's true, though collisions will likely create cones of debris that extend significantly above and below the initial orbits, and those shards

To some degree that's true, though collisions will likely create cones of debris that extend significantly above and below the initial orbits, and those shards could smash satellites in those higher or lower orbits, thus continuing the chain reaction. For example: two satellites collide head-on at an altitude of 300km, creating two debris clouds that extend from 250km to 350km. Each of those catches another satellite, one at 260km and another at 320km, which fragment into debris clouds reaching from 200km to 370km (all numbers made up). Higher orbits, like geosynchronous, have a lot more space to work with and much slower relative velocities than low Earth orbits.