Evaporative cooling of group beliefs does actually work to increase cohesion. If, given a controversial decision like this, people who disagree with it pack up and leave, the variance of opinion within the company will actually decrease, improving cohesion.
I agree that decreeing a divisive policy and prohibiting dissent tends to leave one with a more "cohesive" team. I very much doubt that it's a commercially optimal strategy. You end up with (a) true believers, who build up group think and (b) yes-people and the spineless, neither of which strike me as high value contributors. You've optimized for agreement, rather than excellence, as a hiring criterium.
I would much rather see "cohesion" in the form of a team that, DESPITE widely diverging views, manages to agree on a common vision.
Do you think the Gitlab CEO's policy declaration helped or hurt team cohesion?