from that article it sounds like they are mostly doing "functional partitioning" (moving tables off to other db primary/replica clusters) rather than true sharding (splitting up tables by ranges of data)

functional partitioning is a band-aid. you do it when your main cluster is exploding but you need to buy time. it ultimately is a very bad thing, because generally your whole site is depenedent on every single functional partition being up. it moves you from 1 single point of failure to N single points of failure!