Teardown update! It turns out this logic board has a secret weapon: tiny seals surround each of the cable connectors. (They look like black foam hedges surrounding each of the gold connectors.) What can it mean?
Answer: we think those are waterproof silicone seals. They appear to match a patent Apple filed back in March for waterproofing board-to-board connectors.
This would seem to explain recent tests showing the 6s and 6s Plus to be dramatically more resistant to liquid damage.