For a thin aperture and imaging, well an entire image with off-axis rays, I agree with you.
For on-axis rays, with the elimination of oblique rays in a thick aperture, and forming of a non-image spot, I am questioning it.
I understand what diffraction is, the most common example is a rippple simulation, waves, water for example, hitting an aperture and expanding outwards.
Make that a long cylinder, like a sewer perhaps, and they would bounce off the wall, interefere with itself and propagate forwards, and expand outwards (diffract) as it leaves.
The problem here is that, this analogy does not account for absorbance. The material itself isn't reflective, refractive or diffractive for light on a significant level, it is absorptive. Assume the material is black, and not polished aluminium.
In the case of light, what I imagine, is that as the wave initially expands due to diffraction as it enters a thick pinhole is that once it strikes the material of the pinhole, it will be absorbed, and not reflected back (unlike water) to interfere with itself. As the wave propagates forward, this would eliminate more and more of the outer portions of the wave, leaving a narrower and narrower angle, leaving a diverging wave at the angle of survival exiting the pinhole.
What I'm saying is there is a fundamental difference in the way the material works that is normally unimportant in a ripple simulation idea of diffraction.