It turns out that aspect of the news report was incorrect, maybe someone did their math wrong? I found a copy of the judgement online and it describes his knife as a wooden-handled folding Gerber with a 3-4 inch blade carried in a sheath, unconcealed on his belt.

The judgement also explains prohibited weapon charge. The officer was able to open the knife using centrifugal force and the judge did find that the knife clearly met the criteria for being a prohibited weapon. However, interestingly he found that it wasn't shown beyond a reasonable doubt that Puddy either modified the knife (loosening the pivot) or knew that the knife could be operated in such a way and therefore found him not guilty of the charge. The judge recognized it was an ordinary, legal knife which could have unintentionally developed the characteristics of a prohibited weapon.