How do NFS (and other file sharing) implementations guarantee file stat uniqueness?

The SUSv4-2018ed have the following 2 paragraphs in the specification for <sys/stat.h> header: Note that st_dev must be unique within a Local Area Network (LAN) in a “system” made up of multiple computers’ file systems connected by a LAN. Networked implementations of a POSIX-conforming system must guarantee that all files visible within the file tree…

Details