Let's add a line break here for better readability.

All texts files should end with a line break.

While we're make CS changes, let's import classes like these so we can see the dependencies at the top of the file and have clean class names in the code.

As the pure class name here is not very telling without the preceding namespace (User), I'd add an alias to the import at the top:

use WP_CLI\Fetchers\User as UserFetcher;

Then, the code becomes clear and nice here:

I'd go for something more meaningful here, as ending in an underscore is still "weird" in terms of method name.

For a future iteration, I'd like to suggest splitting these nested function calls to separate lines to improve readability.

			$filtered_args     = array_filter(
				$args,
				function ( $v ) use ( $flipped_user_logins ) {
					// Exclude numeric and email-like logins (login names can be email-like but ignore this given the circumstances).
					return ! isset( $flipped_user_logins[ $v ] ) && ! is_numeric( $v ) && ! is_email( $v );
				}
			);
			$filtered_args     = array_unique( $filtered_args );
			$user_logins       = array_merge( $user_logins, $filtered_args );