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
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[ 4.1 ms ] story [ 32.0 ms ] thread[1] https://en.wikipedia.org/wiki/Blink_(browser_engine)
https://trac.webkit.org/wiki/BuildingCairoOnWindows