Group size caps don't do a lot to limit mass trends. Once a group hits the cap, the administrator will simply open a new one and tell everyone to join that instead. People in only one of the groups obviously can't speak to those in the others, but the administrators can make announcements in all groups, which is enough e.g. for organizing a large event.

Even requiring invitations past 100 members doesn't mean you need some kind of personal connection. Instead of the group QR code, organizers can post their personal one and then add everyone to the group who messages them. It might slow down spammers who constantly try to join random groups, because they won't know whom to ask for an invite.

Lack of global search for groups also isn't much of a hurdle, since you can still globally search for posts inviting people to join a group. It does mean that there's a niche for companies who create groups and assign members to them based on some criteria, as described in the article.

And regarding usernames and anonymity: they work a bit differently than you might expect. Every WeChat user has a unique identifier which is random by default, but can be set to something intelligible if you want to make it easier for others to add you by searching. Then there's a global username, which is shown to your contacts and in groups by default. Per-group aliases don't completely override that; you can still find out someone's global username by looking at their profile. So most people set their alias to their real name in groups that require it and stay pseudonymous otherwise. You can also set aliases for your own contacts, which helps recognize people who constantly change their name and avatar.

I hope that clears things up a bit.