His (and all Googlers) opinion was solicited. His mistake was accepting that solicitation and having a politically unacceptable position.
The context of his memo is an internal gender studies mailing list, which is supposed to serve exactly these kinds of discussions. It's not like he sent this document to unsuspecting employees.
It's hard not to be reminded of Mao's Hundred Flowers Campaign (百花运动) [1] and Anti-Rightist Movement (反右运动) [2].
He wasn't professional because it is not his job to debate this at work! I don't understand how this very simple point gets lost. A public corporation is not a democracy.