Morgans wrote: some stuff about venting.

I'd believe that is the case.
I once had an old, old, oooold house with a kitchen sink that didn't
have a vent. And it was a BIG sink. When you pull the plug on that
granny, it took the wash rag right with it, and me with both feet in
the sink pulling back! The whole sink drained in only 3 or 4 seconds,
and gurgled for a while after that.