No, you're wrong. Herath's straight delivery spins sideways like his normal one. He varies his arm position a little and sometimes cuts slightly more under the ball, which may sometimes land it on the shiny side where it won't grip, but he always relies on chance that the ball doesn't grip i.e. natural variation. The same goes for Shakib's (non-)arm ball, he tends to go more round arm and force it through but it's still chance that the ball doesn't turn.
An arm ball is forced out the front of the hand rather than off the side and is seam up, and swings into the batsmen (if around the wicket) before continuing straight, giving the appearance of drift. It's not often bowled because using natural variation is way easier and a true arm ball is relatively easily picked by a good batsman.
The best example is
Murray Bennett's to Viv Richards, you can clearly see the release in the slow mo. I've never seen Herath bowl one and to my knowledge he never has.