No that is not true. SAN LUNs can be accessed by multiple "users", too.
I don't want to open up a can of worms so I'll give you MY simple definition of NAS and SAN. Keeping in mind that every vendor has their own terminology (regarding luns, volumes, containers, etc.)... typically a NAS serves CIFS (or NFS) shares/exports to users over an ethernet network. A SAN typically uses its own "network" (fabric) and serves block-access LUNs to hosts on the fabric. Fibre Channel is used typically, but ISCSI protocol can also be used and travels through the ethernet network instead of a fabric. LUNs can also be accessed by multiple hosts, but the synchronization for that access (to prevent corruption of files) is dependent on the hosts accessing the LUN.
Since NetApp's FAS/Ontap can do both... I consider it a hybrid storage solution.