It should still work, but only, if you did not enable iCloud Photos on the iPhone.
If the iPhone is syncing the photos with iCloud, the primary storage of the photos is in iCloud the iPhone is just mirroring them. In that case it is not possible to delete the photos automatically after importing them.
It is very risky anyway, to delete the photos right after import, before you can check, if the photos have been correctly without any transmission errors. Occasionally it can happen, that the Mac will crash during the import, before the photos have been properly saved in the iPhoto Library, but will already have been deleted from the iPhone or card. Or, if the cable or the plug are not working properly, some photos may have transmission errors and will need to be downloaded again.
It is much safer to check the photos after the download by viewing them all enlarged, and make the first backup, before you delete them from the camera or iPhone. Keep them on the iPhone, until you have the first good backup.