Source

File: wp-includes/class-wp-query.php

public function is_author( $author = '' ) {
		if ( ! $this->is_author ) {
			return false;
		}

		if ( empty( $author ) ) {
			return true;
		}

		$author_obj = $this->get_queried_object();

		$author = array_map( 'strval', (array) $author );

		if ( in_array( (string) $author_obj->ID, $author, true ) ) {
			return true;
		} elseif ( in_array( $author_obj->nickname, $author, true ) ) {
			return true;
		} elseif ( in_array( $author_obj->user_nicename, $author, true ) ) {
			return true;
		}

		return false;
	}