Is loading non-trigger packages in extensions disallowed now?

After #48533, it appears that package extensions are only allowed to load the host and the trigger packages directly, and other packages (including stdlibs) should be loaded as using hostpackage.thirdpackage or using triggerpackage.thirdpackage. This appears necessary to prevent loops while loading dependencies. Am I understanding this correctly as the suggested way to import dependencies in a package extension? And are packages that are neither imported in the host nor in the trigger forbidden as dependencies of the extension?