Are they actually using multi-master replication? I gleaned from the article series that they have many small independent MySQL clusters, each with one master a

Are they actually using multi-master replication? I gleaned from the article series that they have many small independent MySQL clusters, each with one master and two slaves, and that their worker layer handles requesting data from the correct cluster for whatever shard a request routes to. Writes to a cluster can only go the master. So this seems more like a single-master design, just replicated a bunch but without the masters communicating with each other. That said, I haven't used MySQL much at all. Am I just missing something or misunderstanding what multi-master means in the context of MySQL?