“Yet elsewhere it has been said that huge wood beams like that don’t just burn rapidly; they need an accelerant.”
These timbers were 800+ years old. Have you ever seen how fast a 100 year old house can go up?
I watched a plumber set a brand new house on fire sweating pipe. I’ve also been sweating pipe on my 50 plus year old house right up against wood studs. The wood takes a while to catch fire then goes pretty much out as soon as the torch is pulled away.
Plus, a single piece of wood won’t burn. It must be touching other pieces to really get going. I’m assuming that the post and beams on the roof weren’t laid out like a campfire.