My rule shows it’s using the On Import event trigger which runs after a file is detected in the indexed location in this case.
One possible exception is if you have the indexed folder in a cloud-synced location, e.g., Dropbox. Changes in such a folder may not initiate a filesystem event. However, there are other event triggers, like Before synchronization that can be used.
This is all discussed in the Help > Documentation > Automation > Smart Rules as well as Appendix > Smart Rule Events and Actions.