As reported by plushwork on IRC, get_all_versions_hash() sets a boolean for $usecache which determine whether to use MUC when fetching the list of plugins in a plugintype, however it seems to be reversed:

        $usecache = false;

        if (CACHE_DISABLE_ALL or (defined('IGNORE_COMPONENT_CACHE') and IGNORE_COMPONENT_CACHE)) {

            $usecache = true;

        }

// ...

            if ($usecache) {

                $plugs = core_component::get_plugin_list($type);

            } else {

                $plugs = self::fetch_plugins($type, $typedir);

            }

From my reading of this, we do not use the cache unless:

1. CACHE_DISABLE_ALL is defined; or
2. IGNORE_COMPONENT_CACHE is defined and truthful.

Obviously those values suggest the opposite of the action.