Drink before your thirsty if you leave it until you are thirsty you have left it too late, and it makes no difference if its winter or summer you still have to drink as much its just in the winter you don't seem to be as thirsty
There is a small amount of truth in this, albeit very small and it could have been worded differently IMO.
But basically yes when you feel the need to drink (thirsty) then that it is your body telling you that you need to hydrate as there is a water imbalance (technically this is the start of dehydration)...but in a normal fit person this is a pretty sensitive response and a water imbalance of just 1 or 2% is enough to trigger sense of thirst...so its definitely not too late at this point...its about the right to take a drink IMO
As for the winter vs summer comment...yes again correct if you work your arse off in a winter training and are wearing clothing to keep the wind out etc compared to wearing shorts and t-shirt in the summer chances are the amount you sweat will be the same...more maybe if you are wrapped in a bin bag to keep the rain off in winter and your sweat can't evaporate...the time of the year really has no bearing, it's down to how much fluid you lose when training...the more you lose the more you need to replace...
All this fuss is all thanks to some very clever brain washing and marketing hype...as with everything there is always grains of truth but there really is no need to over think this...its a simple progress, drink when you feel thirsty and eat when you feel hungry. Maintain a good well balanced diet and you will be fine...Everyone should just get out on their bikes and enjoy the pleasures of cycling and not be hung up on when to drink...our ancestors didn't die every time they chased a meal in the baking sun and they had no one telling them how and when to refuel...yet they thought the world was flat