That's not necessarily true. In places with hilly or mountainous terrain, different street blocks may have different signal strengths than the centroid of the ZIP code polygon. If they are using a digital elevation model (DEM) of the terrain, knowing the exact address could be important.
I just went to the site. The only required information to use the website is the ZIP code, and it has options to opt out of mailing lists if you enter in your full address. If you only enter your ZIP code, you will have the opportunity to adjust the spot on the map used for the assessment. Just using the ZIP code option, I was able to interactivley change the spot to my home. It gave a list of channels with distance and direction to the transmitters and the type of antenna needed to receive each station. The angles and distances would be wrong if I just used the centroid of the ZIP code.