California has very strong employee protection. Yes, even though it is "at will" there are still some very strong protections.
For example, in California it is illegal to fire someone for "political activity", which this memo, debateably, may have been.
It is also illegal in California to fire someone for bringing up workplace concerns (this falls under union/worker organizing laws).
Finally, and most ironically, it is illegal to fire someone for bringing up discrimination concerns, EVEN IF those concerns turn out to be false/not illegal/not discrimination.
This guy, debateably, was bringing up discrimination concerns.