I think the honest truth is that a lot of developers see Safari similarly to IE because Firefox and Chrome are quick to jump on and ship new features and "if only Safari kept up" they could be used "everywhere". Combined with the fact that iOS is large enough that you can't just drop Safari support and tell them to use Chrome.
It's a weird world where webdevs apparently ideally just want everyone on the 6 second old version version of Chrome. I totally get it -- it's an absolutely rational stance but it feels a lot like how devs felt about IE6 at the beginning.
You're right, IE6 was amazing at the time. It made a ton of things possible that previously hadn't been, and the standards bodies were lagging behind it. Had MS actually kept updating IE6 and kept it ahead of the standards, the standards would never have mattered, and we'd never have developed this sick taste for IE. No one hated that they had a monopoly, we hated that they had a stagnant monopoly.
I've worked with projects that used iframes in safari. It had some of the weirdest bugs. Some random times it didn't render changes to the DOM. Sometimes when clicking input fields it would focus the surrounding iframe element.
A webview in iOS could sometimes crash system wide. Not enough to restart the app. You'd have to restart the device.
Felt like a sitcom when I had to ask customers if they'd tried turning it off and on again.
Safari is way behind Chrome and Firefox on web features and has all kinds of proprietary rules on existing implementations.
It's not even required or the default install on any OS. And it has rapid updates that constantly test new features and improve performance and security.
Any "exclusive" features are just early testing for later standardization. It's Safari that's lagging in implementing them.
Want to try something fun? Install any browser in iOS and do a test. Then do the same test with Safari. Now ponder why they are exactly the same no matter which browser you install.
If you're referencing Apple's mandate that WebKit be used as the engine for all iOS web rendering, then yes, that's a poor policy ostensibly made in the name of security. Not sure if that's what your point is, though.
Can you imagine how much legal trouble microsoft would be in if they blocked alternative browsers in windows? Simply providing a single default was too much.
It has been argued with various success [1] that Chrome is the new I.E., due to "Chrome exclusive" web standards.
[0]: https://html5test.com/compare/browser/safari-11.2/chrome-30/...
[1]: https://news.ycombinator.com/item?id=16070595