Where it was said that Zetsu is a fusion of Hashirama's DNA and Bijuu's chakra? I don't know where people have been getting that. I thought only his clones are made from that, not the original one.
Zetsu appeared in Part 1, when Akatsuki had no tailed beasts captured. So he can't be made from Bijuu's chakra.
Zetsu appeared in Part 1, when Akatsuki had no tailed beasts captured. So he can't be made from Bijuu's chakra.