Light/aero bikes (and riding practice) will probably make the most difference on steadier 6%-7%-ish climbs - it would be interesting to calculate if the "estimated" VAM/WKG numbers have increased more on climbs like Peyresorude, Arcalis or even La Plange versus something with really steep ramps (that's not a new, novelty-climb) like the Marie-Blanque.
Not putting this as an explanation for everything but even a few years ago it seemed like every team had a special super-light bike for the climbers- even these now have aerodynamic elements and it's no longer unusual to just see teams straight up use the "aero bike" on mountain stages (or like Jumbo when they had Bianchis, just using the Oltre all-rounder bike for everything)