Dips, bench, and overhead presses are compound movements, so of course more than one muscle is being worked. Delts are a synergist in the dip (along with triceps, and a lot of other muscles as stabalizers). But even compound movements have primarily targeted muscles, and its standard convention to ignore synergist and stabilizers in order to talk about movements in a somewhat idealized way.
My point is simply that as the bench angle declines, the chest is targeted more and the delts less, and as the bench angle inclines the shoulder muscles are targeted more and the pectorals less.
But sure - you are right. It is not strictly true to say that the dip is all chest. I guess I should have written "targeting primarily the pectoral muscles, secondarily the triceps, and evolving the deltoids, lats, romboids, and a host of other muscles as stabilizers." That would have made things much clearer, right?
The next time someone says the curl is a biceps exercise, you be sure to correct them by stating the obvious. "If you think curls are entirely a biceps movement, you are sorely mistaken! Don't you know that the brachialis and brachioradialis are synergists and the anterior deltoids, trapezius, scapulae, and wrist flexors are stabilizers!"