Janes, it's not a matter of reasoning. It's a matter not being able to explain yourself clearly. You've provided no explanation of your own for why I'm wrong. You simply point to other sources. I don't actually know if you know what you're talking about or if you're just parroting others. I don't know if you do any of your own work or just blindly read and accept.
Having gone back and reviewed some information I had previously I recognise that my earlier comments were mistaken. But it's got nothing to do with 'how digital sensors scale' or anything related specifically to digital. It's relevant to all formats. And it is, as I outlined earlier, related to magnification. The laws of optics don't change.
What I was forgetting is that the additional enlargement of the smaller image doesn't fully make up for the lower magnification at the image plane. The smaller magnification at the image plane creates CoC that aren't exactly proportional to the difference in magnification. So even though that smaller image (from a smaller sensor or a smaller film format) has to be enlarged to a greater extent, the CoC in the final image are still smaller leading to greater DOF.