I agree that it's not entirely helpful to use "creator" interchangeably for two different things: both the creative team on a particular project and also for, I suppose, more accurately the originators of a specific character/story/setting.
When, to take your example, Ken Niemand and (say) Patrick Goddard collaborate on a Dredd story, they're clearly the creators of that specific story and, indeed, of any new character they might introduce. They don't retroactively get to be creators of Judge Dredd by adding to the existing mythos. That's just not how it works.
It's much the same as the modern version of Wolverine owing far more to the work of Claremont, Cockrum and Byrne than to the Wein/Trimpe guest star in Incredible Hulk #180,* but Wein and Trimpe are the character's creators. Likewise, Swamp Thing has more closely resembled the vision of Moore and Bissette than Wein and Wrightson for decades, but they don't get listed as creators.
(Interestingly, Jamie Delano gets a credit as co-creator of John Constantine for developing the character in Hellblazer, but that was specifically down to Moore asking DC to do that, since he felt Delano's contribution was as important his own.)
*Remember that we didn't even find out that Logan's claws weren't part of his costume until Claremont/Cockrum era X-Men.