Ehhh... while I do consider the "we're all tiny bits of Zodiark" theory a viable one, I'm not going to put my chips on it. References and shout-outs are cool, but to reuse entire plot points... not cool, guys. Not. Cool.

Anyway, we do need more information, and Ramuh is just making a wild guess (or, in Moose terms, putting on his Darksteel Foil Hat). It could be that we're all tiny pieces of Zodiark. Or it could be that Zodiark only exists because of us. Or we only exist because of the darkness and vice versa, so as long as man is around so too shall be the darkness. Ramuh's belief in the inverse (darkness shall linger as long as man exists) is why he passes his harsh judgment, and then says he'll go on a genocidal rampage if we can't prove to him that not all men are evil by enduring his wrath.

My personal theory is that Hydaelyn is somehow an invasive parasite who took over the planet from Zodiark at some point in the distant past, and created us as containers for his essence to keep him weak / sleeping / dead. It explains a significant amount of the non-"generic evil megalomaniac" dialogue of the Ascians. I'm probably wrong on that, but from what information we have... it's the best I can put together.