No, it is a suggested configuration, not the suggested one. And recommending three nodes is not that uncommon for distributed systems, because you cant have a quorum with only two nodes. Some users are more concerned with handling 10s or 100s of servers than having single-server durability.

That said, for most users two servers are fine. Other users don't need any replicas at all since they do a nightly dump from a stored source into mongodb, or they just take regular backups. There are many ways to achieve system-wide durability, not all of them require the database to be durable.