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.
(this kind of thing IMO really added to the utterly arcane set of rules and conventions that makes it feel so inaccessible)
It is good that leaving comments is hard. First off, because it was learned repeatedly, the hard way, that removing that barrier leads to ungodly amounts of literal spam. Second because even insightful comments detract from the main intended flow of using the site, which is: you find a question from a search engine, read the question and verify that it reflects what you're trying to figure out, then scroll down to the answers and learn something. The entire point is to not be a discussion forum (which is also why comments were not threaded for most of the the site's life). In fact, the site came into existence specifically because of frustration with what ends up happening on a discussion forum where people can discuss endlessly.