This is a misrepresentation, before even getting into lack of modern feature support, webkit introduce a lot of long lived regressions on each release and Apple have changed various behaviours in non-standard ways in Safari over the years then left them broken and unpatched on old devices (looking at you autoheight iframes). This has nothing to do with adopting standards and everything to do with a lack of resources, stagnant release cycle and lack of care and support from Apple.
It's not necessary to talk about chromiums bleeding edge non-standard features, webkit easily has the least support for unopinionated standards released over the last decade and adopted by the other two.
> no platform uses the browser more than iOS users do. iOS users always are over-represented in web usage compared to marketshare.
What point are you trying to make here? iOS users have no choice but to use Safari. If you install firefox or chrome or anything that is a "browser" on an iOS device, it's forced to use Safari webengine underneath, so it's essentially safari with different UI, and this is apparent in the useragent string so any decent stats will show that essentially every iOS user == Safari user.