It's a lot like how whenever Apple adds a new feature to one of their operating systems, for Apple and its most dedicated fans, that feature is Apple's invention.

Much like macOS' very official and increasingly nominal affiliation with Unix, Apple's involvement with open-source of more interest to people who see that as a feather in Apple's cap than to people who see open-source as a first-order value of their own. Apple has little concern for either user freedom à la the free software movement or for the collaborative modes of development touted as the primary benefits of open-source by the open-source movement, and this shows in their historical and present actions. For people who give a shit about the goals, practices, and principles of open-source or free software, the creation of WebKit is remembered and remembered as a fiasco.

People who Apple fans first don't know or care about things like 'upstream-first' development, what characterizes a good citizen vs. a bad one in the open-source world, or likely even that companies other than Apple are also criticized by the F/OSS community for engaging in similar behaviors (e.g., the scandal provoked by Canonical's release of Mir after it was developed for a prolonged period in secret).

I never said they created it. I said they have made substantial oss contributions.

And does it really matter? If an OSS project is forked, soars to new heights, and sees use in millions of devices all over the world is that a bad thing? Or would you prefer it remain a quiet little sleepy project for 3 devs to bikeshed on.

Webkit was not a simple fork. Apple took KHTML, pretended to upstream patches for an extremely short amount of time, then purposely made it too difficult for the original maintainers to accept patches any longer until they had successfully killed the project. Classic take what you need from other's hard work at all costs.