Check whether a SearchWP Engine has non-WP_Post Sources

tmp.php

<?php

// Check whether a SearchWP Engine has non-WP_Post Sources.
$engine  = new \SearchWP\Engine( 'supplemental' );
$sources = array_keys( $engine->get_sources() );

$non_wp_post_sources = array_filter( $sources, function( $source ) {
	return 0 !== strpos( $source, 'post' . SEARCHWP_SEPARATOR );
} );

if ( empty( $non_wp_post_sources ) ) {
	// Engine has only WP_Post-based Sources.
} else {
	// Engine has $non_wp_post_sources.
}