The amount of fluid displaced by an object doesn't really
need to be explained.
Whenever you drop or stick something into a fluid, it just
pushes as much fluid out of its way as it needs to. That's
just the volume of the object ... or the volume of as much
of the object as sinks into the fluid.
If the whole object goes into the fluid, then the volume of
fluid displaced is equal to the volume of the object.
If the object comes to the end of its string and stops descending,
or the buoyant force holding it up becomes equal to its weight and
it stops descending, then the volume of fluid displaced is equal to
however much volume of the object went into the fluid. It doesn't
take Archimedes to explain that to us.
What Archimedes discovered is how to tell what the buoyant force
holding the object up is going to be, once it starts entering the fluid.
THAT's going to be the weight of however much fluid it displaces.
