Source

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

public function __isset( $name ) {
		if ( in_array( $name, $this->post_fields, true ) && 0 !== (int) $this->comment_post_ID ) {
			$post = get_post( $this->comment_post_ID );
			return property_exists( $post, $name );
		}
	}