I don’t have an explanation, but here are some thoughts: firstly, these long-term carnivores took quite some time to get where they are; they didn’t cut out salt overnight. Secondly, they are eating a lot more meat on carnivore than they would on keto, and may be getting more salt that way.
Thirdly, no one, to my knowledge, has studied people who’ve been on a ketogenic diet for as long as some of those long-term carnivores. It is not outside the realm of possibility that a long-term ketonian might also gradually come to need less added salt.
Lastly, I suppose we also can’t a priori rule out the possibility that the people who persist in a carnivore diet over the long haul are those whose bodies handle salt in a freakish manner.
The short answer: More research needs to be done on this.