Just a followup to the comment that “Webkit is a Linux/Open Source project”.
WebKit is an Apple originated open source project, based on the Konqueror Linux web browser. So a lot of the original rendering engine work did originate in Linux, WebKit goes way beyond that (for instance, much better CSS support, expended support for HTML5, visual effects that are webkit-specific).
So, yeah, Apple can claim a lot of the credit for WebKit ! I think the main WebKit developers are Apple staff members.