> Then why doesn't it target Chromium as the vastly dominant engine that has ~80% of global market?
Because WebKit sucks!
I don't intend to be vitriolic here, but this really is the status quo of rendering engines. You have Blink, which is capable of making complex webapps and fully-featured browser experiences, and then you have WebKit, which is almost exclusively capable of breaking them. I'm not mad that WebKit sucks though, we need sucky browser engines! Sucky browser engines like WebKit run great on terrible hardware like Smart TVs or WiFi Fridges or portable game consoles that wouldn't be running a browser engine otherwise. The problem is that Apple forces mobile users onto a browser that's behind the rest of the world's web technologies. They can make sucky browsers all day long, and they won't hear a peep out of me as long as I can replace it with a Chromium-based browser. WebKit is holding Safari, and by extension the rest of the internet, back.
That's the problem, and arguably an abuse of market position considering Apple directly profits off the failure of webapps.