I mean, anything is possible, including magical fairies with arson tendencies setting the fire, but I don't think there was any question about Bobby being responsible for the fire. I checked the episode description on wiki and found this sum up - "Bobby tells his confessor how he accidentally killed his family five years before, when, under the effects of oxycodone and alcohol, he forgot to turn off a space heater, which burned his building down." Drunk and high, and breaking into an empty apt to continue getting drunk and high and leaving a space heater unattended which led to the deaths 150 people.... I can't believe he'd walk away with a pat on the back and a "this was an unpreventable accident".
That the building was apparently made of kindling is an entirely different issue, but at the end of the day, all those people including Bobby's kids would be alive if Bobby hadn't been drunk and high in a vacant apartment with a space heater.