Under libel law, opinions are protected from liability for libel.

Anthony Watts of Wattsupwiththat explains:

Opinions are not subject to defamation claims, while false assertions of fact can be subject to defamation. The quote in Facebook's complaint is,

Meta's attorneys come from the white shoe law firm Wilmer Cutler Pickering Hale and Dore, with over a thousand attorneys and more than a billion dollars a year in revenue. They obviously checked out the implications of the matter for Section 230 issues, the legal protection Facebook/Meta have from liability for what is posted on their site. But at a minimum, this is a public relations disaster, revealing that their "fact checks" are not factual at all and should be labeled as "our opinion" or some such language avoiding the word "fact."

As an amateur, it seems to me that if Facebook inserts its opinions into posts or blocks them because of its opinion, then that does make it a publisher with legal responsibility for what appears on its website.

Technically speaking Facebook farms out its "fact checking" to outside organizations, usually left wing groups. In the case of Stossel's video that was defamed, the outside website called "Climate Feedback," which is also named a defendant in the lawsuit.

Watts summarizes well the PR implications:

Such "fact checks" are now shown to be simply an agenda to suppress free speech and the open discussion of science by disguising liberal media activism as something supposedly factual, noble, neutral, trustworthy, and based on science.