I think they are cutting into the steel plate that they welded to it. Doesn't look like they get that far into the actual material. I agree with the point about using the grinder correctly though.
Sure, it doesn't really matter if it is steel or aluminum. They are cutting into the plate that is welded to the material. It still appears to stop once it gets through the plate.
This looks like aluminum billet to me. And other than the grinder stopping, it looks like it’s cutting exactly like aluminum.