It is serious. I have no clear explanation on "why", it's just a taboo, kind of good to know in case you happen to come to the reindeer herding area. I've learned it from basic reindeer herding dog training program orientation; I don't have any reindeer, but I do have reindeer herding dogs. They only allow a few breeds there that naturally know the drill and can survive for hours having fun in savage frost.

Reindeers are so small IRL, about human-size, incredibly warm to touch, and they make sounds like pigs. They are also somewhat afraid of humans, unlike regular livestock, they usually run away from you same as from a dog and sometimes counterattack the dogs, which is scary. Herders would be happy to tell you about their life and everything, but just don't discuss the head count.