The rationale behind disallowing use of marked campsites in the winter is that these sites generally are very poor locations for winter camping. They do not provide good shelter from the wind and as a result of heavy usage are poorly supplied with firewood. Additionally, winter camping can make a mess of a campsite if deep snow prevents campers from locating the fire pit and the thunderbox. Now I realize that the advice you got is that winter rules don't apply until there is a good amount of snow on the ground. However, I expect some judgement could be applied here. If you are encountering unusually cold, windy weather in the late fall prior to any significant accumulation of snow, I really cannot see any ranger faulting you for choosing to camp back in the bush. Actually, the chances of actually encountering a ranger at this time of the year should be pretty small as the interior ranger jobs are seasonal and I would expect they are finished for the year, or if not very soon!
I don't really understand why you would choose to go to the Mew lake campground if winter style backcountry camping isn't available when you could instead hike into one of the backpacking trails. Once you get into late October or early November I don't think you would see very many people on those trails. It's been a while but I have done an overnight hike into Provoking lake in November.