I was thinking the same thing, but on the other hand the whole emperor's haki (or haki in general) seems still so ambiguous definition wise that it's impossible to say. Under a more realistic setting what happened between Zoro and Monet could happen without any mysterious powers; a complete demoralisation by a vastly superior opponent. So, it could be very different compared to Luffy rendering hundreds or thousands of people unconscious. Still, considering lots of "regular" people can learn some forms of haki, Zoro of all people should be able to acquire at least a minor form of emperor's haki because he's quite a menacing fellow, especially compared to Luffy. Unless it's a genetic superpower.
All in all I actually prefer it this way: that we are left guessing and everything isn't revealed immediately. It keeps the story interesting!