To stack or not to stack

Pixel Pooper wrote:

With the same field of view, and the same entrance pupil diameter, you get the same DOF (and the same diffraction) regardless of magnification or sensor size.

I am a bit puzzled by this. Perhaps you can correct my thinking here please.

Suppose I photograph a scene at 1:1 with a full frame camera using a 100mm macro lens set to f/8. (To avoid a complication later, let's assume the f-number is set physically on the lens.) The aperture is focal length / f-number, which is 12.5mm.

Suppose that I now add a 2X teleconverter, without changing the f-number setting on the lens, and move away from the subject so as to get the same field of view as before, and hence the same magnification of 1:1. So the field of view is the same as before, and the entrance pupil diameter is the same because I haven't changed the f-number setting. But isn't the depth of field different now, roughly twice what it was before because the effective f-number is two stops larger, and the effective aperture two stops smaller? If that is so, depth of field does not depend solely on field of view and entrance pupil diameter.

The above statement doesn't actually say that the DOF depends solely on field of view and entrance pupil diameter; it is saying that it doesn't depend on magnification or sensor size. However, being unqualified in any other way it does seem consistent to me with DOF depending solely on field of view and entrance pupil diameter. But perhaps this is reading too much into it, or simply misunderstanding it.

However, if I were to be right about this, would the following statements need some modification/qualification?

Small sensors appear to have greater DOF because at the same f/stop the aperture (entrance pupil) is smaller. With the same aperture you get the same DOF and the same diffraction as a larger sensor.

The deepest DOF is achieved by whatever system has the smallest minimum aperture.

I just did a practical experiment to try to get a clearer grasp of this. I shot this target, with the camera aligned along the white piece of tape so the metal rule was viewed straight on to measure the scene width, with the plastic rule at a steep angle to help assess the depth of field.

I used a Sony A7ii with a Canon EF fitting Meike manual 60mm macro lens set to 1:1, setting the aperture ring to f/2.8, and captured this image.

I then added a lot of extension to make any difference in depth of field more marked. I added a 2X teleconverter, a 1.4X teleconverter and 68mm extension tube. I left the f-number setting at f/2.8. I moved the camera away until I got the same scene width of 36mm and captured the image below. The depth of field appears to be larger even though the field of view and entrance pupil diameter were unchanged.

This practical test seems to me to be inconsistent with depth of field depending solely on field of view and entrance pupil diameter.

I suspect that, as has happened before, I have missed something rather basic. I'm pretty good at that.