I guess it's all in the semantics.
Network Appliance has industry recognition as a NAS vendor providing shared storage pools accessed over IP connections. The other vendors you mention may also have NAS solutions but are recognized as providing Fibre Channel based solutions using block based storage arrays (SCSI over Fibre). Either may be considered as part of a "storage network" but SAN generally refers to block based access and NAS generally refers to file based access. (That's why they are called "filers.")
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This was first published in March 2002