not exactly :
from any damage source (excluding explosions), the invulnerable fighter suffers only 1 HP per FIGHT (passe d'armes), for any amount of damage at least equal to 1.
if you attack twice in a round and hit, then the invulnerable suffers 2 HP loss.
for explosion damages :
you roll 5 dices as usual, but to do damages, you must obtain 6s on dices to inflict the damages to the invulnerable. Apply damages BUT reduce each hit by 1.
Example : a lemure suffer an explosion (damage values 3/2) : you roll 5 dices and obtain 3,2,4,6,6 : there are 2 hits (on 6s) : then apply damages reduced by 1 per hit (2 damages per hit will become 1 damage per hit).
The lemure will also suffer 2 damages, that will be enough to kill it, I think...