I have a feeling our opposed viewpoints here may be all down to the difference between weather in Scotland and weather in Alaska - (ie our snow and ice probably don't compare)

Certainly in typical winter conditions here I can stop way faster with ABS off when on ice and snow. On ice with ABS on I get zero traction, whereas with ABS off I can control direction and braking fairly well. On snow the difference is not quite so marked, as the ABS works a bit, but you are still looking at a couple of seconds difference stopping from 30mph.

It is distinct enough, and consistent enough that I always pull my ABS fuse in winter now. It's just too scary/dangerous otherwise. And believe me, I have tested this extensively, being a devout member of the sideways-driving-is-good religion!

Edit:

It could be down to a similar issue you mentioned with gravel " where locked up wheels could actually stop you quicker because the sand/gravel will then pile up in front of the tires, aiding in the deceleration." as the snow certainly piles up in front of wheels.

That wouldn't explain the improved deceleration on ice - which I'm tempted to believe is down to locking up melting the ice and droppping the tyre through to the surface of the road. Again, this maybe doesn't work on ice more than a few mm thick.