Whose advice? Check out RocksDB's front page: http://rocksdb.org. Empirically what you're saying isn't true in my experience, rather mmap should be used when there's decent coherency w.r.t. the available memory. Without knowing what you're basing your belief on, I really can't address it.
It looks like they are making some claims about OS level bottlenecks specifically with the virtual memory subsystem. This is something I'd like to look into; all I can find is the particular quote but no explanation of where they think the bottleneck actually lies. The experience of, e.g., the SQLite folks seems to be different.
