1. Your eyes need to be open, plus it's not just IR.
2. Apple already covered that in the presentation. They had professional Hollywood mask creator create realistic masks and used that to train the neural networks. I don't think Hackers using cheap masks are going to top the professionals who would have been asked to do their best work. Did you not see the presentation?
This is where the neural processing comes in, seeing as it can not be fooled by glasses or scarves or facial hair, it would also be able to detect subtle facial changes that would not work on a mask, or that's what it would be trained to do.