a pressure cooker isn’t exactly small. this wouldn’t be your typical backpack. pressure cooker would really stick out a lot and be very heavy if full of whatever plus ball bearings. so you’re talking about a backpack looking like the equivalent of several textbooks inside. he’d really stick out in a crowd I think (and likely be bumping people. Several devices? Not likely a 1 man job. And, it seems by not claiming credit, the goal is to add to the FUD. if you set the device in advance, how much time do you have to drop it?
if you’re leaving a bomb somewhere, would you really put it on the curb , where I would imagine you’d be seen by a lot of people and/or cameras?
They do have small pressure cookers with handles like a saucepans. Hiding something under the grate of the tree seems plausible. Wonder if there are photos of other trees and grates...were there also some sort of pots around the trees just for decoration.