It was mentioned when Enma itself was introduced. What do you think is the meaning of "only Oden could ever tame Enma." ? It literally implies Oden tamed Enma and it's haki sucking impulses, just like what Zoro's gonna do.
Mmm yes it does. Mastering it literally means subduing it's haki sapping properties. That's exactly what Oden did with Enma as well, you never see it alone flaring up with Oden.