Though sword does matter but enma isn't amplifying strength just because Zoro is wielding it.
It's bringing out Zoro haki in unrestrained manner. That's why he cut coast and not just tree. So, mastering and controlling enma in fights mean mastering ryou and Boat which results into increase in Zoro's strength and the better he gets, the better chance he has to make blade black
It's bringing out Zoro haki in unrestrained manner. That's why he cut coast and not just tree. So, mastering and controlling enma in fights mean mastering ryou and Boat which results into increase in Zoro's strength and the better he gets, the better chance he has to make blade black