WordPress Username Availability Checker Compatibility

Hello! I am using Restrict Username along with WordPress Username Availability Checker. Restrict Username itself works amazingly. However, WordPress Username Availability Checker uses validate_username() core WordPress function for validating username. It seems the Restrict Username is not using the filter provided by it and using something else to generate the error.

Is there any reason for not using that filter? Is it because that does not allow returning a custom error?

What function should WordPress Username Availability Checker use to validate the username? They said they will add it to their compatibility.

Thank you so much!