It doesn’t only come from supplements, but it’s how most people get it these days. Livestock aren’t sticking their faces in wild dirt and flora very much, so most need supplements. Also even with livestock being given supplements there is a surprising amount of B12 deficiency in the omnivore population, arguably more should be supplementing.
The rules state that only cards with listed probabilities (green text, affected by the dice) will count. Which I don’t agree with but is consistent. Perhaps there should be three modes for RNG instead of being a binary yes/no. Anyway, the fact that RNG is such a narrow category helped me get this one in two guesses.