One could say that a company serving 10 million is a megacorp.

I don't believe that's the question being asked. Rather they are asking whether the process was/is only viable with the support of a megacorp? Could this same process not be just as effectively used by smaller geographically distributed facilities run separate orgs or corps?

That is plausible. It depends what you mean by "megacorp." If what we mean (in contrast to artisanal craftsmanship) is just industrial scale operations - then yes, we certainly can organize industrialization differently than megacorps. Smaller sized corps, State-owned enterprises, co-ops, federated mutualism, and more, are all possibilities. But if the suggestion is that we could support modern population levels with just traditional production methods - that probably is unlikely. To steelman the argument, maybe proponent could suggest something like advanced permaculture. I'm skeptical that would work, but something along those lines is a possibility.

What's empirical about that though?