A couple of things we know and some speculation:Well in the videos I've heard them say "pouring 1200* F into an ant hill" but I suppose it is likely that they just looked up the melting point and put that number.
It seems like if it was near the actual melting point, most of it would be starting to solidify by the time they were pouring, and whatever came out would be 'blobby.'
1. We know the aluminum is near its melting temp because as it pours, it is forming a solidified, thin foil skin.
2. Heat of fusion for most materials is very large, so even as it starts to solidify, it takes a large energy outflow to solidify a little bit.
3. It is therefore likely that as the aluminum flows into the anthill, it coats the walls of the tunnels with a thin layer of solid aluminum, which helps stabilize the tunnels while providing pipes for the transfer of more molen aluminum.