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.
That Apple locks down iOS very tightly is a separate problem.
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.
Firefox does not use WebKit (and at this point neither does anyone else), but they can look at patches to WebKit to understand how Apple improved Safari's performance. Because Apple produces the hardware, the OS, and the browser, the choices they make in the WebKit implementation are highly informative.
> it shows how being FOSS isn't enough
This is true for every app on iOS: even if you have the source code, you can't necessarily run a modified version, and so you're missing one of the main user freedoms that FOSS should guarantee. I wish Apple would rethink this, but Safari is still usefully open source.
Just because you can't literally copy the code from one place to another doesn't mean you can't learn from it. I'm not a lawyer, but my understanding is it's fine to learn what series of system calls they're making and look at their optimizations.
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.