when KH1 was first released, Nomura didnt even know he was going to do a second game, so he didnt have the idea of the Nobodies yet.
here's how i figure it:
a Nobody is the body and soul of a being that has lost its heart but is strong willed enough to stay 'alive' (for want of a better word), but their lack of a heart makes then nonexistant.
the Heartless are, as Yen Sid puts it, Darkness made real. perhaps they are just manifestations of the darkness in people's hearts, and as they emerge, they almost literally eat the heart of their 'host' (again for want of a better word) and consume more hearts to increase their numbers.
hope that helps.
EDIT: another solution is maybe the character(s) that said this (the only ones that come to mind are Leon and his gang) were mistaken for whatever reason.
with all due respect,
Merlin326