[topaz0]

This effect should be negligible. The whole point here is that it takes way more energy to take a chunk of ice and turn it into water (at fixed temp -- namely the freezing point) than it does to heat that chunk of ice a few degrees (below the freezing point). And remember that you don't just have to cool the cream to its freezing point, you also have to remove enough energy to overcome its latent heat of fusion. If you were doing this just with the heat capacity of ice from like -20 to -5 C, you would need many times more ice than you could make ice cream. Like tens to hundreds of times. The blog discusses some related facts a bit near the end.