Photo import prompt when iPhone plugged to Mac?

I would describe this as an offer to allow you to import photos, with a notation that you have already 4,223 items and have no new items to import. (If there were new photos, there would be another section displaying them.) The action buttons are grayed out, since no action is appropriate.

Just above your upper red oval, there's also an option to have the Photos application open automatically when this iPhone is plugged in, in case you have reason to want that, perhaps some day in the future when your phone cannot upload to iCloud. (I'm not recommending mixing two transfer methods, by the way. It will cause confusion eventually.)

Photos sees your iPhone plugged in and has no way to know whether you did that to charge the phone, to back it up or sync it with Finder, to use it with Photos or Image Capture, or for some other reason. It shows you the options it can offer.

If you don't want to avail yourself of any of those options, just click over to the Library view or your favorite view and ignore that entry in Devices.