I don’t see any issue with the uninstall besides the fact that echo a string(this interpreted us failed) and using hardcode prefix.
Below code have been tested locally with the latest version of WordPress.
// Access the database via SQL
global $wpdb;
$wpdb->query( "DELETE FROM {$wpdb->prefix}posts WHERE post_type="acme_product"" );
$wpdb->query( "DELETE FROM {$wpdb->prefix}postmeta WHERE post_id NOT IN (SELECT ID FROM {$wpdb->prefix}posts)" );
$wpdb->query( "DELETE FROM {$wpdb->prefix}term_relationships WHERE object_id NOT IN (SELECT ID FROM {$wpdb->prefix}posts)" );
// echo "UNINSTALL DAMMIT!!";