Oh I guess I didn't recognize the context fully. I have no disagreement with that (noting that I have little to no knowledge of the effectiveness of it either). You're talking about a serious life-threatening medical condition where a particular diet may have specific desired effects, and even if there are subtler or longer term negative effects, that can be well worth it. It's very different from general nutrition for a healthy lifestyle, which is what I though you meant.
The things you pointed out are "non-dairy milks, meat substitutes, breakfast cereals, and one type of nutritional yeast". The first three are manufactured products (and probably have B12 mostly because it is added as a manufactured supplement, meaning you are not getting the other associated nutrients which would normally be found with B12 in real food), and the third is exotic which is probably also manufactured in a sense. It's certainly not something you can just hunt, scavenge or forage and eat.