The way I read it, it only applies to stack installations according to my instructions.
Stack installation should always be done parallel to the vehicle floor-do not install at an incline, such as leaning against a vertical surface, as this will invalidate your warranty."
It really doesn't matter much for me since I bought my 2 PDX's off ebay and you have to buy from an authorized dealer to get a factory warranty. With what I saved from an autorized dealer price, I could buy another amp and come out ahead. I have a 4.100 and 4.150 and they were under $700 to my door. Crutchfield would be over $1200.