Right-click would be equivalent to long-press on iOS. But don’t long-press too hard or you’ll get 3D Touch. Or maybe you do want 3D Touch, so you press hard. Surprise! Your iPad doesn’t support 3D Touch; you were thinking of your phone. Does this particular app even support 3D Touch? Only one way to find out!
Inconsistent and not discoverable, which isn’t anything like a right mouse button at all.