I think Keto can seem like a cult to an individual.
If you can’t stop talking about it or thinking about it and you continue Keto even if it is not working for you (due to a genetic issue or gallbladder issue for example) then, it might be very much like a cult for you.
Just as some vegans will ignore weight gain, bad blood markers etc. because they were raised in a vegan family - pretty much a cult-like behavior to stick with something not working.
Another example might be cycling in and out of Keto. Ancestrally we may have been in Ketosis a lot of the time but when a large grove of easy to obtain fruit presented itself in early summer, when people were running around doing things in the good weather (needing extra energy) and while the animals (having just come out of winter) were not really worth the effort to hunt and kill (being unfattened) - people would likely have eaten the fruit.
Someone with a Keto-cult-like mentality would immediately try to rip apart that idea. Instead of accepting the example and trying a couple of weeks of whole food fruit and some starchy vegetables after 10 or 15 weeks of solid Keto (to see how they react - positively or negatively) they will instead refuse. Even if they feel dragged down or psychologically burned out on Keto - they’ll keep going (potentially downhill) without a break.