He is not kidding. The substrate is what the display is built upon. In the image below, the layer labelled 8 is the substrate.
Code:
Legend Item
1 Upper layer
2 Transparent electrode layer
3 Transparent micro-capsules
4 Positively charged white pigments
5 Negatively charged black pigments
6 Transparent oil
7 Electrode pixel layer
8 Bottom supporting layer
9 Light
10 White
11 Black
Since the eInk substrate is not in the light path, it has no effect on the display. Please also note that item 1 labelled as the upper layer will have various layers above it such as the light guide, capacitive touch screen, matte surface layer, etc.