Well, it would have to begin with defining "self-ignite", and if you mean that to be anything other than arson, collision/rollover, or exposure to other fire, then yeah, I'd agree with you.
The only actual stats I can find that break this down are incomplete, though.
They don't cover 100%. Mechanical is 49%, electrical is 23%, intentional is 8%, collision/rollover is 3%, and exposure to other fire is 5%. Those percentages combined for a total of 88%. What are the other 12%?
The above link also states "In 2003-2007, U.S. fire departments responded to an average of 287,000 vehicle fires per year.". This does not match to this report, on the same site: https://www.nfpa.org/News-and-Research/Fire-statistics-and-reports/Fire-statistics/Vehicle-fires/Highway-vehicle-fires
This report states that in 2003 the number of fires responded to was 286,000 in 2003, and dropped each year to 2007 (227,500). That means the average reported in the first report at 287,000 can't be right. Unless all the stats by year are wrong.
Arson