That is not correct.
A Necromancer is a Sorceror/Spellcaster/Mage/Wizard who specialises in Necromantic magic.
All Necromancers use Necromancy. Not all users of Necromancy are Necromancers.
Nice try, but no. It's disingenuous to cherry pick something and then use it to try and constrain my answer.
How about you answer this: If Necromancers were the same thing as death knights, then why does Death Knight have it's own page with it's own list of known Azerothian Death Knights none of which are on the list of known Necromancers? I'll tell you why: Because they're two different things (even if they share some common traits and abilities)
https://wow.gamepedia.com/Death_knight