If a skeleton shoots an arrow through some lava/fire or uses a flame bow, the flaming arrow can explode TNT even if /gamerule mobgreifing is set to false.
How to reproduce:
Spawn in a skeleton and trap it.
Place a block of TNT nearby
Place some lava in between the skeleton and the TNT
Switch to survival and stand near the TNT until the skeleton hits it.
This is a situation that is not prevented by Mob Griefing turned off. I believe this works as intended.