That doesn't stop their need to work around the bugs in the existing version. It's easier for the jQuery maintainers to work around a Webkit bug than to submit a patch and hope that it gets applied in the next few years.

EDIT: I misread the article on how the bug was actually handled, and that it was fixed five years after it was reported, not that it took five years to apply the patch. However, it still is easier for the library maintainers to use a workaround than to patch the browser itself, especially when legacy versions still need to be supported.