It would, but that's not what's being discussed.

The accusation is that the companies are restricting speech by saying that managers cannot provide a reference, even if they clearly state in said reference that the views are their own and not that of their employer. Nobody is trying to compel anyone else to provide a reference, the idea is that nobody should be prohibited from it, especially since for long term employees leaving on good terms their manager(s) is/are probably one of their best references.

Edit: and remember, this is to file a suit, not win a defamation suit, which can be difficult. However it is very expensive for an employer to defend itself, easily tens of thousands if not six figures of dollars. How the game is played is a disgruntled ex-employee files a suit and then tries to settle for low tens of thousands of dollars, a portion of which goes to the employment attorney they hired.