The problem with this line of reasoning is that Google can make a legitimate claim that the firing was for cause. Creating a hostile work environment is sufficient cause for termination, and I am quite sure that something like this is buried in some clause of the standard Google employment contract.
You don't have free speech rights at work, or more to the point you can say whatever you want and the company can do just about whatever it wants in response to this speech. To go full Godwin here, if this employee had, instead of talking about how women and minorities "lowered the bar", wrote and distributed interally a manifesto about how Google had too many Jewish employees and how these employees were mostly hired because of post-Holocaust western guilt and maybe we need to reexamine whether this whole Holocaust thing really happened I can assure you that he would have been shown the door within the hour. No one is locking him up for his speech act, but Google would have had no problem in firing him for that same speech act.
Others have pointed out in great detail all of the unpleasant future consequences for any potential interaction that leads to conflict between this employee and his peers. This situation was created entirely by the employee who posted the manifesto. He can sue Google, but he will lose.