Correct. Jax-Ur embodied the "Evil Kryptonian" trope way before Zod, even if Zod - technically speaking - was created a bit before.
In the Silver Age, Jax-Ur was a bald Kryptonian with a moustache whose experiments had destroyed one of Krypton's moon; he had been the first prisoner in the Phantom Zone, too (this detail was changed in recent years, when Morrison made Xa-Du the first Zoner - maybe to give the Phantom King a specific, Zone-related set of powers). He was the most recurring Zone criminal, followed by many other Zoners whose powers vaguely differed from him.
It should be noticed that most of the Phantom Zoners, Jax-Ur included, didn't even have a specific characterization. Zod's most defining traits came from Superman II and Terrence Stamp's over-the-top, flamboyant style. From that moment on, Zod became THE "evil Kryptonian" (even if visually speaking he was a bit different from Terrence Stamp) and Jax-Ur was more or less forgotten. His only remarkable reintroduction was in the DCAU, but this version of Jax-Ur, which is the same version reused by Waid, was basically Zod with a different name (even if in the DCAU comic book tie-ins they actually reintroduced a version of Zod:
https://dc.fandom.com/wiki/Zod_(DCAU))
When Johns took over with the the Last Son storyline, Zod was made more similar to the Superman II version, but in one specific panel Johns also reintroduced Jax-Ur as the first Zoner (and the explosion of Krypton's moon was supposed to be related to Kandor's disappearance, even if this plot line was later discarded). In the New Krypton storyline there was a more substantial reintroduction of Jax-Ur, this time as a one-eyed scientist rather than a soldier. This specific version was abandoned until Bendis somehow reintroduced him (with two eyes though) in his Rogol Zaar storyline.
The most bizarre aspect in World's Finest 18 is not that Waid reintroduced Jax-Ur (it is a typical Silver Age concept after all, and we all know how much Waid loves classic characters), but that he decided to use a specific version of the character that has never been used in the comic books before, and probably the least interesting one. I guess that Waid wanted to pay homage to Silver Age Jax-Ur, but the deliberately chose not to revamp him visually and simply decided to follow the DCAU version, which is the least obscure one today.