Solution
This spec does not necessarily mean you can't have a gap between VIH and VIL shorter than 400 mV (actually I've tested and it worked without errors), however it is the minimum spec where we can guarantee the performance.
Use case scenario: generating a clock. It means you can't generate a clock with an amplitude <400 mV. With "can't" we mean guaranteed, this will depend heavily on how good of a clock the customer needs for their application. This is related to the ringing/distortion margin the application can accepts since at higher frequencies you'll start noticing distortion:
However, this is also related to the cabling and accessories the customer could use.