Fundamentally they can do it because partitions are relatively small, so you'll have hundreds of partitions from hundreds of different tables on a single machin

Fundamentally they can do it because partitions are relatively small, so you'll have hundreds of partitions from hundreds of different tables on a single machine and a handful of hot partitions are only a tiny blip. (There is of course a limit to how hot a partition can be, they'll cut you off before you start consuming a large amount of resources on the host.) Unfortunately single-tenant systems are more constrained in this, because you don't have millions of partitions to spread across thousands of machines.