A GFCI works, in part, as you describe below.
In addition, it detects a ground/neutral short
as described at the following url:
http://www.codecheck.com/gfci_=ADprincipal.htm
I believe that function is what the poster was
referring to.

Correct. Modern GFI's really have two parts. The main one keeps you
from killing yourself, by cutting off current when you have a hot to
ground current. The active SECOND part (a second toroid) allows the
device to detect ground-neutral faults, so it can trip then, when its
sensing function has been compromised.

SBH