Unfair competition is, particularly in California, which interprets it's UCL very broadly.
Let's set aside all other claims (there are others), and take a look at "Unlawful, Unfair, and Fraudulent Business Acts" under that.
We'll also throw out unlawful business acts (I don't see anything unlawful so far).
Unfair:
"An “unfair” business act or practice, as defined by the UCL, is typically committed by either a company or a business competitor. ... In the context of a business competitor, it is considered an unfair business act when the company does something that broadly undermines competition in the marketplace."
Additionally, they consider " immoral, unethical, oppressive, and unscrupulous" business acts to be unfair.
Banners on wp engine sites probably not a good thing under this. Threatening your competitors with bad keynotes unless they pay you, also probably not a great practice.
(I do think you'd be fine to say they suck. I just don't think you can get away with basically extorting them)
Fraudulent:
"The UCL also prohibits “fraudulent” business acts or practices, which means any conduct that misleads or deceives consumers."
Note that it does not have to be defamation, or malice, or illegal. Just misleading or deceptive. More exactly, it does not have the elements of common law fraud - Intent is not a requirement, and negligence can be a violation.
So pure opinions without intent or with negligent intent that actually deceive consumers, while not defamation or common law fraud, are quite possibly a fraudulent business practice.
Overall, I think they have a stronger case than you might. Not on defamation, but on other things.
Regardless of the outcome, the approach i see taken by the Automattic CEO here seems remarkably stupid.
Don't mix your roles unless you want a court to mix your roles.
When he threatens to ban them from wordcamp[1] in what capacity is he doing it in?
1. Which, btw, the central website totally avoids mentioning who is in charge or paying overall anywhere i can find. I hope it's not the foundation (or him or automattic) and he's not mixing roles further while threatening his competitors.