Why do I have a vent stack if none of my fixtures are vented?
Your system is almost certainly vented. Plumbing vents provide a path for sewer gases to escape so they don't bubble up through the P traps into your house under pressure (the sewer gases will expand and exert greater than atmospheric pressure), and the vents also keep water pressure from siphoning your traps dry, which would create an easy natural path for sewer gases to infiltrate your home.

So the comment about "wet venting" is right on target. As long as the combination of pipe diameter, the slope of the pipe and the distance between the trap and the vent ensure that the pipe doesn't run full of water (creating a siphon), a single downstream vent can be just fine.