yes. it does. it's not directly harder, per say, but it does work better. The cold causes the particles to form stronger and more denser bonds - allowing for a seemingly harder piece of hdpe that slides better. A good example of this is cheese. If you leave cheese out on the counter (before it gets all moldy), it's relatively soft. It is still a solid though, and it still holds it's brick form, however, if you were to try and cut it, it would be easy. Now, stick that brick of cheese in the freezer for 2 days, and then take it out. It's still a solid, and it's still in it's original brick shape, but you'll have a much harder time cutting it with that knife. It's exactly the same.
And to reply to the post with the liquid nitrogen, that's because it is in extreme colds. Everything has a point of the extreme. The reason it does this is because it is instantly changed from normal temperature to an extreme low, and it doesn't adjust properly. It's the same reason you don't take a hot pan off the grill, and immediately set it into the cold water of the sink to clean it - it ruins the pan. Or same reason if you take a turkey out of the freezer, you let it thaw for a long time before you start cooking it - so that it can gradually get to the proper temperature rather than instantly.