I don’t grant your claim that a diet isn’t optimal just because it includes a b12 pill. In fact that it’s so trivial to supplement makes me wonder why you take for granted that there’s some magical bonus from incidentally getting it from a non-supplement.

It’s like saying getting vitamin D fortified milk is superior to taking the fortification yourself.

I bet this cashes out into the appeal to fallacy. Frankly I see no reason why the optimal human diet can’t be some undiscovered 100% artificial diet that we can synthesize from scratch. To say otherwise, including your b12 belief, leads to some pretty hilarious reductios.