That statement is a lawyer vaccine.
Any product of combustion for complex molecules (diesel fuel, roofing tar, over-cooked hamburgers on the Bar-B-Que) results in new molecules forming that can be carcinogenic. It is all a matter of amount and length of exposure. Eat 500 overcooked burgers a day for a couple years and you'll have in increased incidence of cancer. The EPA regulates materials as carcinogenic with little regard to typical exposure levels.
Just don't breathe diesel exhaust the way laboratory rats do (24 hours a day at choking doses), and you'll need to worry more about meteors than diesel exhaust. The human body really has an incredible ability to filter, clean out, and repair damage from dangerous molecules.