California's statues are about protecting an individual from retaliation against workplace complaints. While you can argue it is related to FoS, I think it's a bit of a stretch. The statues would not, for instance, protect you if you told your co-workers your boss was murdering children in his office every day, no matter how much you believe it to be true.
Anti-SLAPP is about protecting people from the financial drain of a lawsuit by a third-party looking to intimidate. It does nothing to force anyone involved to continue to employ, or do business with, any individual.
The problem in this case, from an employer's perspective, is that they have one employee who is creating a toxic atmosphere not from a protected position, but from a hostile one.
And while you might believe that people who read his memo as "toxic" are over-reacting to it (and there is an argument there, though one I wouldn't agree with), they must be free to "overreact" and speak their mind.
It does get very, very tricky, since while unpopular racist/sexist opinions may necessarily create problematic dynamics in the work place, that could easily also be true of opinions those who decry this memo would think should be protected. If I work at a company with a steeped culture of faith, and I walk around talking about how Christian's are stupid, to what extent should the employer be forced to continue to employ me?
I tend to believe that, to the extent we have freedom of speech, we chose when and how to exercise that in public. Our choices have consequences, and it is on us to accept those.
I'm a small business owner. If tomorrow, I write a "memo" which says something as moronic as "Women don't make good clients", and I HAVE women as clients, I can't expect them to continue to want to do business with me. I can't demand any type of legal protection which would force them to continue to do business with me.
It is just as true that, at least with my company, I can see many types of clients who would not appreciate the libertine culture of my agency. I've had more than one potential client, who found out we used to take our employees to a regional Burning Man event, show extreme discomfort at it, and it likely cost me the account.
I've had job applicants turn down working here because they were more conservative (and one I remember who thought we were too conservative for them).
That's just the nature of the thing. People are free to say what they want, to associate with who they want, and must accept the social consequences for doing so.