what is this?

I don't know that cable could stretch far enough to render it inop'.
In cases where the adjuster is inop' the adjust paw will wear the cogs off the adjuster.
A much more common fault ( other than simply put together wrong ) is the tiny little neck on the cable guide that is held in a hole by a return spring. That little neck needs to be present and properly installed or the cable will appear too long and adjust paw will not function properly.
Just another example of a small detail having a large effect.