Words are not defined by their creators, they are defined by their use.
Those people indeed know what USAGE is -- in fact, they define what USAGE is. When we talk about USAGE, we talk about how people use it.
They might not know what "USAGE as defined by coldtea is" but that's not really relevant to them or to the population at large.
:-)
use: the action of using something or the state of being used for a purpose.
usage: the action of using something or the fact of being used: a survey of water usage