I see that the fork is of WebCore rather than WebKit. Since both are LGPL any forks would also be opensource but with forking WebCore rather than WebKit the thing that uses WebCore doesn't have to be open.
Why exactly is WebKit so important? It underperforms Blink and is forced on iOS devs. The majority of its users only use it by default. It’s Apple Internet Explorer
I see it as a solid competitor to Blink, and JSC as a competitor to V8.
How much of that competition rides on the fact that Blink isn’t supported on iOS?
A bit, but unlike many others I support the decision. Browsers are beasts, and I don’t trust anyone to port one securely to iPhone at this point