An incendiary device is a bomb. An incendiary statement is, "You're ugly, fat, and stupid." Both are likely to produce an explosion of one kind or another.
Incendiary means more than flammable. It means explosive. If you are a radical who changes the world by exciting people and makes as many enemies as followers, you might be called an incendiary figure. The speeches you give that rile people up are incendiary. People who set fires are sometimes known as incendiaries but more often are called arsonists.
