But Ian, your examples neatly (and gorgeously!) illustrate my point- you have big foreground elements there and the compositions carefully use elements that tie foreground to background to build in a compositional sense of depth. Perhaps we can agree that this is something that must be done regardless of focal length / FOV to communicate a feeling of depth.
Consider what you sometimes see in panos: mountains or seascapes with no foreground. Then there is a tendency for the long horizontal to dominate and there is little to draw the eye in and experience the depth of the scene. And if you try to put a foreground element in there and are shooting a wide lens, that foreground element can become so enormous in the frame relative to whatever is out at infinity. Even the largest mountains or big thunderclouds or big sunsets can be diminished to a long thin line in the pano frame by an ultrawide lens. Then if you have a daisy in the foreground... what is the shot really about?
When I went out west I thought I'd get a lot of use out of my wides, so that is primarily what I packed. There is the tendency to see a scene and the whole atmosphere is so gorgeous and unique that you (or I) want to capture the whole thing on one sheet of film. But of course it doesn't work that way. The lenses I used most out west turned out to be (usually) the longest I'd packed.
When you say longer lenses flatten things out, I think you are referring to 'compression' of foreground against background. To me, that is not flattening at all, that is juxtaposition, and depth can be built into a scene if its composed properly. In the shots you posted, I see some care expended to preserve depth, i.e. not to incorporate so much foreground as to blockade the main subjects.
I think another possible point here, and one seldom used in pano, is limited DOF. If the foreground and background are in equally tight focus then I would argue that a sense of depth and scale can be lost. Not always but there is a tendency to go for front to back sharpness that may not necessarily be the right approach. I suppose that with ultrawides this is is a stronger tendency.
I need to go back to Macchu Pichu, I only visited once when I was 7. I hope there isn't a McDonald's there now.