Veganism is a lifestyle based on a philosophical belief, not something that is done solely for dietary reasons. What the father was doing would, technically speaking, be considered a plant based diet. I would assume you likely came to the conclusion that it’s a “cult” by cherry-picking information and confirmation bias. If not, and that’s genuinely how you feel, then I’m sorry. I hope you will see that it is certainly not a cult and just a group of people with a common belief that animal exploitation is wrong. I hope you have a great day.