I'm not sure what your point is.
>Any B12(-like) substances that are in plants (e.g., spirulina) are unlikely to be in a form that's bioavailable to human beings.
Yes, this is true.
>Vegans have to take B12 supplements of some sort or eat foods that have been fortified with B12. That's just a fact.
Modern vegans do need to take supplements, yes. The question is whether vegans in the ancestral environment would have gotten sufficient B12 from eating unwashed vegetables and roots. As far as I know, research is lacking on that point, but it is plausible.
