In fact, it's the shock wave that is more deadly for a grenade than the shrapnel. Gotta love Mythbusters
Uh... no, not even close.
Actually, you're both wrong - and both right.
It depends entirely upon the type of grenade being used.
A fragmentation grenade carries less explosives (therefore less overpressure - smaller shockwave) but its case is designed to come apart (fragment) and create a projectile hazard of relatively large and thick pieces of metal (or sometimes plastic).
Concussion grenades are designed with more explosives (three to five times more than a fragmentation grenade, for a significantly larger overpressure/shockwave), but have thinner cases creating a smaller volume of less dense fragmentation particles.
For example, a US Mk2 fragmentation grenade has 2oz. of high explosive.
A US Mk3 concussion grenade has 7oz. to 8oz. of high explosive.
Of course though, none of that has anything to do with thunder.
The Player's Handbook describes the Thunder damage type as "A
concussive burst of sound..."
By the rules,
Deafened is a Condition, not a Resistance or Immunity.
So, a Deafened creature still takes damage from Thunder.
In the real world, thunder damage (the concussive force generated by lightning) is described like this by Dr. Raymond M. Fish in the book
Medical and Bioengineering Aspects of Electrical Injuries:
"A mechanical shock wave produced in association with the lightning can produce tympanic membrane perforation [eardrum] and
internal organ contusion or perforation."
In other words, internal tissue damage. Even though a shambling mound may not have organs, it still has tissue that can be
contused, otherwise it wouldn't take damage from bludgeoning weapons either.
One might say though that one cannot have lightning without thunder, and shambling mounds are immune to lightning. However, it's actually more than just flat immunity; shambling mounds are created by and
healed by lightning. So, one could make the case that the concussive damage of a lightning strike is offset by the healing that takes place when a shambling mound is struck by lightning.
But thunder by itself? Yes, it would take damage.
So, as Patrick McGill said in the second post of the thread:
No they aren't immune to thunder damage, just as deaf people in real life aren't immune to shock waves.
He's absolutely right.