Thank you, that is most interesting.
So maybe what feels like souplesse might be something else, more flexible in the rear triangle, either vertical (springy) or lateral (whippy). Or maybe it is souplesse as the function was described., but not at the toptube but in that rear triangle

I am here to learn.
And sometimes theory "dictates" a certain systemic way of thinking, but in practice it might all be different.

By the way, I often wondered, what if chainstays would be more wide and lateral stiff, but vertically more compliant? I recall a recent discussion about it, and the bikes like that, that do exists, weren't great rides.