Kel'thuzad is credited with being the creator of the Abomination, it's not assumed.
And nobody ever said they've invented Necromancy, originally, Dreadlords experimented with basic forms of Necromancy back during the War of the Ancients and Gul'dan also dabbled into it (and Ner'zhul was frankly in contact with Gul'dans creations).
They were the ones that really went neckdeep on necromancy, but never the ones that invented it.
Let's not forget one thing, this whole necromancy thing makes little sense in Maldraxxus, because dying in the Shadowlands = Perma Dead; no ressurection or raising you from the dead possible, not to mention that seemingly some souls are able to choose their form (like Vashj or Mograine)...so why do seemingly some souls choose to be skeletons or require stitched up bodies?
It's not like somebody raised them from the dead, because that's not possible.
The archetectual style was taken from the Nerubian, which were by their own accord very skilled builders and an oldass civilization to boot, it again, works that those have a developed a certain style.
It's not fucking assumed lore if you know Pre Shadowlands lore, it's in there and nobody ever questioned it because it worked.