The world is full of religious people, Trump supporters, conservatives, and people who agree that men and women are biologically different and that might affect their life choices. Adults can go to work and collaborate productively with these people.
Google is quite clearly now full of people who are not adults in this respect, and who feel it's totally OK to refuse to work with someone they never even met because of a memo they wrote on political and policy issues. And their management supports them in this.
From the companies point of view this may mean they are justified in firing him, but California law apparently disagrees - you aren't allowed to fire someone for their political affiliations, and that is true even if other employees are refusing to work with them. That would require you to fire those other employees, as they are the ones refusing to do their work unless a legally impermissible act is taken.
if he was a white-supremacist who wrote a scientifically-backed manifesto about biological differences with his black coworkers, would you still be blaming the people who are 'shunning' him?
No, how they act is their fault.
The firing is justified from the point of view of the company, they want someone that can contribute, and he's just effectively removed himself from that pool. In my opinion, this is a bigger part of why they fired him, and the fact that they disagreed with his policies was just a cherry on top.