Tim,
what ed wrote is correct. To say it in simple words: If a NDMP vendor writes a tar, a dump, or something completely proprietary is up to the vendor. Also if the vendore is using tar, it does not mean in any way, that u can restore it with a standard tar.
The NDMP protocoll is telling the NDMP Filer: "I want you to backup this filesystem". It also tells the NDMP filer to do it to an attached tape, or over the network to another NDMP host. Also the filer transfers meta informations to the backup server. Thats it.
So the communication protocol is defined, the content of the backup stream is not.
Compare it to http protocol. The communication is defined, the content of a file download for instance can be everything, http does not care.