I’m guessing that the pressure cooker acts to keep the explosive gases in check up to a point so that when they finally blow there is more force involved. Maybe this is super obvious to all anyway but I dunno, maybe not.
Same principle as pipe bomb and smokeless powder firearms. There needs to be pressure to have an explosion. Broadens the range of materials that are suitable as a "propellant." Might even work with matchheads.
It is a variation on a pipe bomb with the advantages that it will hold a larger volume than a pipe bomb and doesn’t look like one either.
If you found a pressure cooker in a backpack would you immediately think “It’s a Bomb”!!!
Low explosives would include black powder (nowadays black powder substitute) or smokeless powder. The latter will only explode if confined and permitted to build up pressure.
Guess they’ll be going after rifle and pistol powders next?
Alternately, the air-tight seal could contain the odor of the explosives, so bomb-sniffing dogs would not smell them.
Maybe the pressure cooker prevents the gunpowder scent form leaking out, so that it’s not detected by bomb-sniffing dogs.
When I was a kid, we shook the gunpowder out of about 100 dud firecrackers and packed them into a cardboard box about the size of a cigarette package and lite it on fire. Quite a fizzle and short burst of fire and smoke. Thank God we didn’t have access to a pressure cooker.