Kessler syndrome is only a problem if the satellites are in LEO. They don't have to be.

They do have to be if they want to be approved by the FCC.

And btw Kessler syndrome applies to any orbital band. You've got the logic backwards. Kessler syndrome is usually only considered a threat for LEO because that's where most of the satellites are. But if you're throwing million(s) of satellites into orbit, it becomes an issue at whatever orbital height you pick.