A double boiler is different in that the steam that heats the top compartment does not contact the food. Basically you’re roasting it in a sealed compartment at around 212 degrees. A pressure cooker with the chicken in an inner bowl would steam it, but the results would probably be pretty similar.
What I mean by dry-steaming is I braise pork shoulder in the pressure cooker with nothing else but salt, no liquid beyond what it throws off itself. That would probably be a terrible way to cook chicken.