It is a complete sentence. It's a sentence that leaves out a lot of detail, but it has an object, a verb, and a subject (as much as any sentence with an indefinite subject has a subject). That object, net, refers to another sentence and usually the two sentences are written without intervening punctuation, but nonetheless this can be viewed as a complete sentence.
I know of a single instance in canon in which an 'e' or net gets used without the speaker uttering the previous sentence to which it refers:
'e' neHbe' vavoy That isn't what my father wanted.
(Literally, Daddy didn't want that.) (Star Trek VI)
This sentence is not a good data point, because The Klingon Dictionary tells us that when you use neH want in a sentence-as-object construction, you don't use 'e' or net, yet here it is.
Check this out:
qama'pu' DIHoH net Sov One knows that we kill prisoners. In English, that is a conjunction used to introduce a subordinate clause. In Klingon, net (and 'e') is a pronoun, not a conjunction. It refers to the first sentence, and is used as the object of the second sentence.
qama'pu' DIHoH We kill prisoners
net Sov One knows that
So it's not entirely clear that one can just say net Sov all by itself. However, members of the KLI have (non-canonically) been saying net Sov all by itself for decades, so I would bet that if Okrand were interrogated and asked about net Sov and similar sentences, he'd say they're okay.