There are orders of magnitudes between players, ratings and comments. Most people do not comment, nor rate. So you would maybe have 1 comment, 10 ratings and 100 users. The only public interaction is the comment. And about half of the ratings are seen in the global feed for like 5 minutes.
The potential public exposure for the other 94 people would be unexpected. And most of the 5 people with public review probably did not read their settings.
You mentioned the usernames are accessible via cookies
I did no such thing. I said web games can recognise recurring users. Not that they do this by the account name. I believe this to work with cookies.
Yes, itch.io don't have all the features other social networks have but I think is implicit that any interaction you have will expose your username. Like creating a comment on this post, creating a devlog or publishing a game.
It has basically no social network features at all. Itch is a download store that happens to also host some web games and happens to have a rudimentary commenting system.
Providing multiplayer support in any way would be nice for the platform. But it would also be a nightmare to implement.