Scalability: by default, non-relational databases are split (or “shared”) across many systems instead of only one. This makes it easier to improve performance at a lower cost.
That statemenent can be read in the introduction. Are they meaning “sharding” ?
But this is not a default, for example Atlas does not use it. So I get confused, what does it mean?
The only way you can share data between systems in MongoDB sharding or replication. But replication is explained right below, so this should refer to sharding. Or something else.