I've been told this as well by well meaning beekeepers, although I did note that they had higher than average losses during last winter, based on what I understood to be UK averages.
But to be fair to an increasing number of beekeepers over here, they are starting to abandon the Open Mesh Floor, and just returning to an ordinary floor, therefore I would assume (from a novice's point of view) they would then have very high condensation levels in their hives, hives which are generally not insulated on the walls, only above the crown board.
So, I'm guessing that the way your hives are able to deal with no top ventilation is that you have an OMF, but what is a beek to do if they go back to closed traditional floors (and yes before anyone says it, I know that OMF reduce the Varroa numbers a bit, etc. I've read the research too).
Everyone I've spoken to about this, is in agreement, better a cold hive than a wet hive - the thinking is that the cluster will maintain the necessary temperature for survival, but water dripping down on them will kill them... however isolation starvation is often mentioned in this context.
