If you're going for "realism" and consistency with other elements of the game, I'd recommend for hand/m30 grenades to swap them back to Lethal/Smashing, and I will explain why.
Most common hand explosives do not kill via the heat they produce, while they do produce some heat and sometimes can even set fires, that's only a minor side effect from the explosion and not the cause of most of the damage these weapons can inflict.
Explosives do damage to people via two primary mechanisms:
A high pressure blast wave which causes internal injury. (Smashing)
High velocity shrapnel that can cause injury and bleeding. (Lethal)
If you are close to the explosive, you'll catch both but the blast wave will inflict a lethal injury milliseconds before the shrapnel gets you. Further out, you catch mostly shrapnel. However, in COH most grenade powers are applied directly to the face of the target.
M30 having knockback is another indicator that this attack deals smashing damage.
As to why this change back would be consistent with other elements of the game?
I present the Incendiary Grenade power with the tags (Ranged) Targeted Area of Effect, Fire, Damage over Time, Fire.
https://cod.uberguy.net/html/power.html?power=v_paragonpolice.grenade_launcher.incendiary_grenade
This power is used by a couple npcs and is also available as a temporary power.
I posit that the original design was correct for the M30 and other similar grenade powers, and changing them to have a fire element does not appropriately represent the powers being changed due to the game having an incendiary grenade already.