My advice to someone who does not want to buy a pressure cooker is to BUY A PRESSURE COOKER. All kidding aside... It's a MUST HAVE
The science of WHY lies in the difference between sanitizing and sterilizing. If you boil something for a "really long time", all you do is clean it really well.
When a product is "sanitized" - glasses at your local bar, they are brought up to a minimum rinse temperature of 180F. If this doesn't happen, certain bacteria remain and the local health department will slap the bar with a violation.
When you go to a surgeon to have your gall bladder removed, the knife that cuts you needs to be "sterilized" or you risk getting a contamination/infection. This process requires a minimum temperature of 250F. This temperature can only be reached by steaming "under pressure". Google AUTOCLAVE for the long version on this.
When you sterilize a substrate you effectively kill EVERYTHING alive. When you then introduce mushroom spores, there is nothing else in the substrate to compete with them for survival.
The $100 you spend on a pressure cooker will guarantee that the other $100 spent on spores, substrate, casing, jars, isopropyl alcohol, plastic baggies and a mister isn't wasted.
Results like this will not happen outside of a sterile environment. Seriously, get a pressure cooker and you'll be SAVING money.
Peace