Well, you're wrong then. You clearly don't understand what it means to be a practitioner of Necromancy and therefore a Necromancer. Simpy put, the degree to which Death Knights make use of necromantic magic is not sufficient to classify them as practitioners of Necromancy.
The term practitioner implies a lot more than simply having some knowledge of an art, it entails a significant degree of expertise. From vocabulary.com: "A practitioner is someone who has learned everything about his or her field and is actively working in that field."
Death knights have an innate mastery over the dead, but they don't study the art of Necromancy. They aren't Sorcerors.
Typically the word "practitioner" is applied to the medical field, so that lends itself to a nice analogy:
A medical doctor - by definition - is a practioner of medicine
Paramedics, first aiders, nurses, attendants, homeopaths or pharmacists all have some medical training, but they are not medical doctors.
Other good real world analogies would be the difference between a mechanic and a mechanical engineer,
or an electrician and an electrical engineer,
or an interior decorator and interior designer,
or a draftsman and an architect,
or a sailor and ship's captain
etc etc etc.
Simply put, Death Knights are categorically not Necromancers even though they may have some skill in the arts of necromancy.