Defamation has long been deemed to be speech not protected by the first amendment. Because of the risk to freedom of expression, such claims have been limited in numerous ways, and are exceedingly difficult to win. But where a plaintiff does manage to overcome those hurdles, it is not an indicator of the fragility of the first amendment for the defamation claim to actually succeed. It’s simply applying well defined outer limits to freedom of expression, no different than ones for fraud, false advertising, and other malicious conduct that incidentally involves speech.
Two points @grellas mentioned are key.
First, this case involves falsehoods. Factually untrue assertions are generally outside the scope of protected speech: http://law2.umkc.edu/faculty/projects/ftrials/conlaw/lying.h.... (Otherwise, the first amendment would swallow claims for fraud, false advertising, etc). This case, moreover, doesn’t fit the scenarios where false statements can be protected expression, such as hyperbole or satire.
Second, the speech was against a private party. The first amendment restrictions on defamation claims are less stringent when it comes to private parties than to the government, politicians, or public figures. In particular, where defamatory speech turns out to be false, it might still be protected by the first amendment if it was directed to a public figure and the false statements were not made recklessly or knowingly. But if a private party is involved, that additional layer of protection doesn’t exist. (The theory being that there is a greater interest on making statements about public figures, where sometimes you might get the facts wrong, than with private figures.)
I’m a first amendment extremist, but a defamation claim here doesn’t strike me as problematic any more than a prosecution for fraud. There is no legitimate expressive value in spreading demonstrable falsehoods about a private party. Nor is there a slippery slope. The key elements of private party versus public figure, and demonstrably false versus possibly true statements are bright line limits that have long served us well.