No myth at all Peter. For a given aperture and camera, a fast scope will produce more signal than a slow one. The aperture determines how many photons get through from any point in the sky, but the focal length (and hence the Fno) determines how large an area of sky feeds photons through that aperture and into each pixel - for a given aperture and camera, a faster scope looks at more sky, resulting in more detected signal in each pixel. In more general terms, it is wrong to say that FNo by itself is the only determinant of image SNR quality, but it is equally misleading to contend that aperture by itself is the only determinant of image SNR. As Rick points out, sampling must also be included.
Re Stan Moore's paper, ask why the two images he presents - from f4 and f12 scopes of the same aperture - have the same plate scale. Unfortunately Stan doesn't explain this, but I assume that he got there by binning 3x3 in the f12 configuration. That would give about the same scale and also restore the large SNR loss inherent in operating at f12 vs f4. If he used that approach, the only thing he ended up losing at f12 was ~90% of the field of view, but in the process he did not show the large drop in SNR at f12 vs f4 (with the same camera and no binning).
If anyone needs an illustration that aperture is not all that matters, try imaging with a 2x Barlow in the imaging train
Peter, your observation that you get better SNR results from an f5.5 scope than from an f5 could come about if you use different cameras on the two scopes.