Wow, okay, I seem to be on a roll with running into dumb issues with Hammer. This time, though, it’s a bug, as opposed to my own incompetence.
The Detonate input on Hammer’s satchel charge entity (grenade_satchel) doesn’t actually work - the charge doesn’t detonate. Hammer doesn’t display any arguments for the input, and firing it manually in-game doesn’t make the charge explode either. I’m assuming this is a bug, since there don’t seem to be any other special parameters for the entity that would relate to making it explode.