compact

Flushes, then compacts until no level is over its budget. Returns when the tree is in shape.

The rotate is what makes the first half of that sentence true. Maintenance only ever sees sealed memtables, so without it a store whose active memtable had not reached StoreOptions.memtableMaxBytes would compact whatever was already on disk and leave every recent write in memory — which is not a barrier, and not what the sentence above says.