Less oxygen in there, when filled with a liquid. Be it raw water in summer or AF in winter. I firmly believe that almost all corroding of blocks and manifolds is done, when in cold storage and without a liquid medium in there to reduce the oxygen and cold corrosion. Perfect conditions for it.
Winter storage means 2 things, cold and dry air, both of which are ideal conditions to inhibit rust, not encourage it.
Previous boat was a 33 year old 4.3l when I traded it off, current is only 20 years old. Engine looks like new with all original parts yet, including manifolds.
My routine has always been the same, pull the plugs, shove a screwdriver in there to clean out, pull the thermostat housing, and dump some RV antifreeze down to flush. The plugs stay out all winter.