I don't want to shatter the mystique or anything but "determining relevence" for a simple keyword-based query is easy-peasy: all pages that contain all of the keywords are relevent. Job done. Now, ranking them perfectly, i.e. determining which pages are likely to be the most relevent is considerably more complicated.
Now, can we apply the same easy-peasy logic to determine if one website is relevent to another? Maybe. But what you certainly cannot do, is use any algorithmic logic to reliably determine that one website is not relevent to another.
Now, if only someone would tell Google this, we could all get on with our lives.