LOL, no, that's not how it works..
Hunter didn't need to prove he wasn't using, the government has to prove he was "using", define what that means, and/or define "addiction", put that definition on the form and prove he was "addicted".. plenty of people use drugs and don't believe they are addicted, whether or not you think they are addicted. Even then, if they don't believe they are addicted, there is no definition of addiction on the form, then they aren't lying on the form. They have to prove he actually lied, as that's what he was charged with, as opposed to trying to game the system within the loose rules and definitions.
As far as I'm concerned, using means that while he was signing the document he was simultaneously hitting the crack pipe, or at minimum, high on crack, and that needs to be proven beyond a reasonable doubt. The form needs to define what "using" means. Like, currently using or currently on the substance? How many hours/days/weeks ago does it reach?
This is just another case of the prosecution and the Judge convincing a jury that he was guilty of something they couldn't prove. Hopefully it gets overturned.