We're obviously both contributing true things.
I started with the claim that there is nothing that necessitates a 1:1 mapping between sensor elements and tricolor raster pixels. And I think the multilevel Foveon Quattro challenges our ideas of what a paradigmatic sensel is.
I think you agree on this much. You would say that Foveon Quattro has 19M "pixels" and not 29M. So you would count only the first level sensors as "pixels" but not the second and third levels. So there is no necessity for a 1:1 mapping in your view.
But I also claim that nothing necessitates that the number of final output pixels be equal or less than the number of sensels. It is an empirical question: a contingent matter of fact, not a necessary one. The number of output pixels might be greater than the number of sensels.
This is especially the case here because the digital photography sensors, of those that we know so far, commit one to confabulation, unlike in classical Nyquist theory. As such, judgments of being "believable" or "convincing" enter into the picture.
And for some reason, Sigma claims a capability of 39M pixels from this sensor. Why 39M, and not 38M or 40M? What is it a function of? Whatever the case, I suspect that there is something both convincing and believable about those 39M pixels in just the way that attempting to derive 39M pixels from a 12MP Bayer sensor would look neither believable nor convincing.
For historical reasons, I dug up Bayer's patent, and took an interest in the way that he named his sensor elements as luminance-sensitive and chrominance-sensitive. I do not take this as a counterargument against the idea that they are also "pixels" in some practical sense, so we're not in real disagreement here. The key word here is "practical".
Sometimes organizations adopt "standards" or institutionalized definitions as a practical way to regulate discourse in an active area of research and development. In some cases, these definitions do capture something essential about the subject of the definition.
In many cases, however, nominal definitions are applied to subjects that do not admit a nominal essence. And in some cases, a merely stipulative definition is used for purposes of social regulation, sometimes for political reasons. Unfortunately, often the stipulative definition offers no truth value; it is simply what we call a "nominal kind".
For me, the ISO definition of "pixel" falls into some of these areas. It is stipulative, though it does capture something of the subject. It is there to regulate discourse. It is clearly not authoritative in any scientific sense, nor immune to revision. Maybe this is the best it will get.