We had to change Firefox so it could be automated with WebDriver BiDi. The Puppeteer team had to change Puppeteer in order to implement a WebDriver BiDi backend, and to enable specific support for downloading and launching Firefox.

As the article says, it was very much a collaborative effort.

But the announcement is specifically about the new release of Puppeteer, which is the first to feature non-experimental support for Firefox. So that's why the title's that way around.