I'm not sure if someone has mentioned this already, but if someone has gone down the path to allow enumeration (username check during signup, which is immediately returned ton the user), I would say this is a strong argument where you could protect yourself against enumeration more easily..

The amount of times a user will likely type their username/password incorrectly is going to be a lot higher than those who attempt to re-signup for an account. Therefore adding additional protection to the signup page (hard rate limiting, bot-protection) then aids to stop this username/email enumeration without the need to be as strict on a login page. This is also compiled with ip-based rate limiting/bot protection that can get very frustrating for users within a corporation that have single public IPs for all their users.