WP the_content and REST requests

<?php

    /**
     * Assign class methods to hooks.
     */
    public static function init() {
        if ( ! defined( 'REST_REQUEST' ) ) {
            add_filter( 'the_content', array( __class__, 'filter_content' ), 999 );
        }

        add_filter( 'rest_pre_dispatch', array( __class__, 'rest_check_referer' ), 10, 3 );
    }

    public static function rest_check_referer( $result, $server, $request ) {
        if ( null !== $result ) {
            // Core starts with a null value.
            // If it is no longer null, another callback has claimed this request.
            // Up to you how to handle - for this example we will just return early.
            return $result;
        }

        $referer = $request->get_header( 'referer' );

        if ( 'www.test.com' !== $referer ) {
            add_filter( 'the_content', array( __class__, 'filter_content' ), 999 );
        }

        // Otherwise we are good - return original result and let WordPress handle as usual.
        return $result;
    }