I have heard for decades that 99 percent of forest fires are started by lightning. Are you saying that you have recently heard the other claim?
From Smokey Bear Guidelines:
Statistics showed that nine out of ten of the fires were person-caused and, thus, preventable. With this in mind, in 1942 the Forest Service organized the Cooperative Forest Fire Prevention Program with the help of the Wartime Advertising Council. The Wartime Advertising Council was composed of people experienced in the business of advertising who donated their talent to the U.S. Government to get important messages to the people.
Posters and slogans were created through the Wartime Advertising Council, including "Forest Fires Aid the Enemy," and "Our Carelessness, Their Secret Weapon." By using catchy phrases, colorful posters and other fire prevention messages, the CFFP Program encouraged people to prevent accidental fires and help with the War.
... PS: Granted, the Smokey Bear emphasis was on carelessness, not malice, however the point remains that, especially today, we have every reason to believe there are enemy agents behind some or even many of the wildfires.