Definition 1. - your version "a worker is only "worth" $50/hour if there is a supply of people willing to pay them $50/hour."
Definition 2. "a worker is "worth" $50/hour if they add $50/hour value for the business." (who then goes on to pay the worker as little as possible, maybe only $5/hr)
Not sure which definition is "correct".
Definition 1 includes both supply and demand. You have a worker willing to work for $50/hr and someone willing to pay them $50/hr.
Definition 2 is only taking into account demand. You have a supplier who has set their price, but no buyers at that level.
This doesn't seem like a particularly helpful illustration.
Another way to look at it is: what is the maximum a company could conceivably pay for a particular role, even if that amount was way above the market rate? That number would presumably be a bit below the value that role is expected to contribute.