Though some Nexus devices haven’t received the Nougat update yet (and some won’t at all), they’re still getting the security update. That includes the Nexus 5 and Nexus 6, so don’t get too excited if you see this update pop up.
If you want to download the factory images, hit the source link. You could also wait for the OTA to appear on your phone, which is easier to update if you have the patience to wait. Let us know when you get the update!