Agreed, it does sound that way, but it's amazing how many anecdotes there are of people who eat exclusively 1 type of food and thrive on it. I don't think modern intuition about nutrition is likely to stand the test of time.
The conclusion I have come to is that humans, when starting from an over-fed modern baseline, are robust enough to eat a totally shit diet for a couple months. This is also long enough to convince us it is worth blogging about.
The subculture of long-distance hikers who optimize by going fast and light (maybe 1-3 lbs food per day plus 10-20lbs of gear). They'll go for months on extremely weird diets that optimize for calories per gram while doing more exercise than they've ever done and generally be fine.
I've only heard that about carnivore diets, and it makes sense to me. Where can you find all the nutrients necessary for a mammal to survive? In the body of another mammal, of course.
You can pick one of the usual Native American crop patterns and get a solid set of vitamins. Potatoes + beans + squash or something like that, maybe with some corn. Cf. Mann's 1491. If you're going for a minimal veggie ingredient diet these new-world combos work well as a base, in part because potatoes are pretty much a superfood.
Sweet potato doesn't contain B12 - you're probably thinking of Vitamin A.
Dairy doesn't contain enough B12 to supplement you on it's own, which is why the study recommends against and instead suggests taking an actual B12 supplement (Puritan's Pride lozenges)
4 weeks shouldn't be enough time to develop a serious B12 deficiency but doing this for longer could impair you cognitively.
fwiw, I wouldn't personally be a massive advocate of supplements - dietary sources are usually better if possible - so not sure whether the study's supplement recommendation here is a good one. Just quoting the instructions given to participants
Potatoes are a good source of vitamin C, B6, potassium, magnesium… Butter has vitamin A, D, E, B12, K2, etc. This diet doesn’t seem TOO crazy. Perhaps pairing it with a multivitamin supplement wouldn't hurt though.
I wouldn't quite say "most" as potatoes are surprisingly nutritious, but yes, it is notable that the article doesn't contain the words "nutrient(s)", "nutritious", "vitamin(s)" or anything similar I could think of.
I've always been curious whether many of these diets lacking appropriate B vitamin requirements might have a compounding effect w.r.t. people's interest & willingness to continue trying such diets...