A pressure cooker is a poor choice for an explosive shell, compared to other options. The maximum velocity of components from a bomb is dependent on the pressures generated during bomb's explosion/combustion. High explosives are engineered to generate the highest possible pressures for a given weight of explosive. Gunpowder is capable of very high pressures, but a pressure cooker is a poor delivery vehicle.

A common pressure cooker, in order to be a truly effective weapon, needs to have it's safety valves defeated, and the lid welded to the saucepan.

Let's take a .45ACP cartridge. In order to send the bullet downrange, pressures of ~20,000 psig are generated by the gunpowder. Or a .300 Magnum. In order to send the bullet downrange, pressures of ~60,000 psig are generated.

A pressure cooker's safety valves start releasing pressure at about 25 psig, and the lid's connections generally fail around 200 psig. This is orders of magnitudes less than what's capable in gunpowder. Which is why a pressure cooker is a poor choice.

And one of the deformed lids was found on a roof? Only possible if the pressure cookers were set down with the lid up. If this person had set it up on the side, more than 3 people would have died. It would have sent most of the shrapnel, the lid, and most of the explosive force into the crowd.

This goes to show that not much technical sophistication was involved in these bombs. Everything technical is pointing to garden-variety amateur.