Literally everyone on the site is permitted to propose an edit, and everyone with at least 2000 reputation can make unilateral edits. The proposals are approved by a 2 out of 3 majority of random unilateral-edit-privileged users. None of this is considered "moderation" and is not done by "mods". Of millions of Stack Overflow accounts, only a few dozen have ever actually been moderators, and they do a tiny share of curation. Their main job is responding to flags.

> It should have been very costly for them to make a new users feel unwelcome.

The overwhelming majority of people who came to the site wanted the site to be something that it was fundamentally not trying to be, and often something it was fundamentally trying not to be. It was correct to make such users "feel unwelcome", because experience has shown that they typically cannot be reasoned with or explained to. The statistics make it clear that most of them never had any intention of trying to join a community (or, say, ask another question after the one that motivated account creation) in the first place.