Imagine an alleged arsonist going to court, and his defense was "not guilty your honor, I didn't put all that fuel in one place, I just flicked a tiny lighter!"
This is a poor analogy. Besides the distinction that arson is intentional rather than negligent, arsonists burn things that would not have burned otherwise. While PG&E's negligence sparked the fire, the brush in the area would have burned at some point in the near future regardless of their actions. The Twitter thread mentions the fire that started in the area around the same time because a tree fell, or some other inciting event could have happened. While PG&E should be held accountable for failing to uphold legal standards of inspection, the Camp Fire or a similar one was likely even without negligence, so to compare PG&E to arsonists is misleading.
Negligent vs intentional is orthogonal to responsible. You could be responsible but just negligent, like an idiot lighting up a cigar at a gas station. You could intentionally light a fire but not responsible, such as if someone tricked you into doing it or you were poisoned with hallucinogenic drugs, or just plain crazy.
None of these corner-cases do anything for your arsonist analogy. Whatever else one thinks of the utility, it is not plausible that either it or its employees intended to start a fire.
