The same way we can have a genuine conversation about the best web stack to use if people with a certain "undesirable" opinion get fired for expressing it.
(I don't think the fact that other Google employees supported the document is strong evidence that you can think critically and reach the same conclusions. If Google reached one mishire in James Damore, they almost certainly reached many others. If you listen to the YouTube interview with him that was flagkilled off the front page earlier today, he says he was recruited for his puzzle-solving skills and ability to code; presumably Google recruits lots of people that way, and none of them have ever been examined for critical thinking in the interview process. I can certainly attest that at no point in my Google interview earlier this year was I asked to do anything that evaluated whether I could combine a couple of sources and reach a defensible conclusion and defend it, which is a pretty common engineering skill.)
The very meaning of "advancing harmful gender stereotypes" is that he said things about gender that were so wrong that they have only the most tangential connection to reality and would seriously harm the business if time were spent to even demonstrate that they're wrong. That's exactly the same reason that if I advocate for Classic ASP on NT Server, you don't spend the time and effort to set up a test network and benchmark if IIS is getting you better performance.