As I said, that statement in bold is treated as axiomatic around here. Where's the proof? Given that the motion of our arms is isolated from our (effectively inflexible) skeletons by flexible connective tissue, what evidence is there that narrower bars increase pressure on ribs and lungs?

There might be some minimal effect of the connective tissue interacting with the muscles of the back, I guess. But I've spent many years riding handlebars from 38 to 42 mm in width (and maybe wider; I usually just used whatever bars came on a given bike without measuring them), plus various aero bar configurations, and I've never been able to detect any restriction of breathing.

(With one exception: low aero bars that put me in a nearly flat-backed position shove my guts up against my diaphragm, restricting breathing from below. But that has nothing to do with handlebar width.)

Again, counter-evidence: professional riders, who ride with maximum efficiency or risk losing their jobs, clearly are willing to ride with narrower bars, rack up the kilometers on interminable brutal climbs with their fists next to their stems, etc., etc. Why would they do that if it put them at a disadvantage?