I guess the average person probably doesn't know about flashpoints etc. I don't really understand what this means.
I don't really know how fast a fire would spread. OBVIOUSLY it would be utterly utterly stupid and reckless to set one and I'm recoiling in horror even at the thought of doing this, and I think most people would, it's not even in the realms of "mistakes anyone could make". But I think that it's definitely plausible unfortunately that someone could be stupid enough to believe that a fire would spread slowly enough to rescue people. Especially if your main "experience" or "knowledge" of fires is things like TV soaps, where the USUAL storyline is that a fire starts and you think everyone is doomed but then, miraculously, everyone is rescued and smoke inhalation is never a problem. We all know soaps aren't real life but if it's the only place you've ever seen something happen then you can develop a false impression regardless.
You see it all the time on threads where people speculate "Oh in this <dire situation> I would do <implausible thing>!" Obviously most people would never even dream of deliberately setting up the dire situation. But I think the average person probably does have a false assumption about the likelihood of being able to rescue things from a fire. You see it in the hypothetical scenario of "What would you save from your house?" and the way people act in fire drills - very very slowly, and wanting to take all their shit with them.
Which is, I think, why they didn't get convicted of murder. Even though the assumption was wrong, it's likely one that many people hold. It's the fact that most people would never set the fire in the first place that got them convicted at all. But it probably couldn't have been for murder because of that "reasonable doubt" thing.