Google's Pixel phones don't have all of what Apple calls "Image Signal Processor" (ISP) done on the main SoC. The 845 and 670 have an upside in that the modem is incorporated into the main SoC (where Apple needs to allocate board space to a discrete cellular modem). That allows Google to put in a discrete Pixel Visual Core. If Google pitches large amounts of camera work to the PVC then dropping down to 679 won't make a huge difference.Taking nothing away from the acknowledged superior speed of Apple's A13 chip, much of that sounds like what's been written about the Google Pixel 3, hampered as it is by the Qualcomm Snapdragon 845 and "only" 4 GB RAM. It's also interesting that Google's $399 Pixel 3a with a less-capable Snapdragon 670 and also only 4 GB of RAM is considered the Pixel 3's photographic equal. The Pixel 3 will shoot 4K video at 30fps, not 60.
The other issue is that Qualcomm isn't trying to keep their image or DSP improvements out of the 600 or 700 series — perhaps a bit smaller but are getting the same upgrades the 800 series when new (not a simply process shrink).
It appears the Pixel 4 will also have some supplemental computational chip that does more than just PVC does (perhaps running parts of Project Soli).