"There is not tortles in this world" is a campaign premise that is incompatible with tortles. That's it, it doesn't need any more justification.
I'm sorry, but calling that a "campaign premise" is...I have no words. That's not a campaign premise. It's not even a
setting premise. It's literally just "I declare X!"
A "campaign premise", like a "story premise", is the central conceit or set of conceits around which the experience (campaign, story, show, composition, whatever) turns. Like the story premise of Harry Potter is, "What if there were a secret school for wizards and an unlikely nobody ended up going there?" Something as small as "this world doesn't have species X" is not a premise. It is simply a ban because the GM declared that it would be a ban.
Or, if I may, let me put it this way: The campaign premise is going to be either a thing you sell the players on, e.g. you include it in the elevator pitch,
or something revealed through the act of playing the campaign and discovering the true situation at hand. If the GM gives you the elevator pitch, "This world doesn't have tortles", would you consider that a meaningful campaign? If the GM promises you revelations to come, and the revelation is "this world doesn't have tortles", would you be satisfied with that as the point, the draw, of the experience?
If not, then I don't see how anyone can argue that that is a campaign premise. A campaign premise either draws you in, or is the payoff for sticking around. The absence of various races isn't a campaign premise of
Dark Sun, for example, it's just a choice the designers made. The highest, most succinct expression of the campaign premises of
Dark Sun would be: "What does heroism look like in a world where mere
survival is a pitched battle, and evil rules the shattered remains of the world?"
