To understand device agnostic as a philosophy we can look to the word agnostic itself as a descriptor of the word device. In this case we are saying we do not know what the device is that is accessing the page. In this case it is not that we cannot know, we are not seeking to know — we are designing with no particular device in mind.
Quote:
Disruption will only accelerate. The quantity and diversity of connected devices—many of which we haven’t imagined yet—will explode, as will the quantity and diversity of the people around the world who use them. — Future Friendly