I think the point (or assertion) is that what already happened won't make the damage from the new hit be worse than it would have been otherwise.
To put (fake) numbers on it to clarify the assertion:
Let's say that JG took 17 points of damage last week. The assertion is that if he takes a hit that would cause him 5 additional points of damage if he had 0 damage points it would still only cause him 5 additional damage points even though he's operating with 17 points of damage.
That's not true in general. For some injuries a hit that would cause 5 damage points to a healthy player would cause, say, 11 points of damage to an already-injured player.