And it's a plausible threat. The fires, or some of them, may in fact have been set by terrorists. But a wildfire is not like a bomb going off in a nightclub, or a hijacked jetliner crashing into a skyscraper. It's far more likely to simply be a natural disaster.
If someone keeps shooting out your car windows, and your tires, and your pets start turning up missing or obviously killed on purpose, you would start to suspect that someone really doesn't like you. But when a wildfire burns down your house, you have car accident, your wife gets cancer, and your cat gets leukemia, it's certainly possible that someone doesn't like you, but there are other, more likely, explanations. Unless, of course, you receive a letter that makes demands, and offers proof of culpability...