Actually Biden said he thinks the guy was drunk. That was his opinion. He probably believes it was true.
There probably was evidence that led him to that conclusion. Just because a person can pass a breathalizer test doesn't mean that he or she wasn't drunk at the time of an accident (Just ask "twelve-hours later" Cheney.) And what USED to be considered legal is now considered drunk, which means that our understanding of what constitutes impaired driving has changed with more evidence.

"Every gun that is made, every warship launched, every rocket fired signifies in the final sense, a theft from those who hunger and are not fed, those who are cold and are not clothed." Dwight Eisenhower