Earlier quoted context omitted.
On the other hand, Safari's WebKit being open source allowed Google to build Chrome on the same engine. Years later, Chrome's Blink engine being open source meant the same for Opera, Brave, and Edge. It is hard for individuals to run and maintain custom forks, but the benefits of open source in terms of preventing lock-in are substantial.
The same WebKit that now breaks the mobile Web and it being open source is completely irrelevant.
WebKit on Mac continues to be usefully open source, and I believe performance improvements that Apple has made for Mac Safari have been incorporated into Firefox, Chrome, etc.