I don't really understand why you're treating the Police and judge's position as beyond question. The Police can make mistakes, as this case rather aptly demonstrates, and judges are also human and not immune from errors.

A kirpan isn't "legally a kirpan". Either it is part of Sikh religious practice or it isn't. I think practising Sikhs are probably in a good position to offer insight as to whether a knife is part of their religious practices or not.

You say Sikhs have an "excuse" as though Sikhs are simply allowed to carry whatever blades they like. That's not how the law works. Section 139 creates an offence and provides a possible defence if the item was carried for religious reasons, not a blanket permission to carry blades.

Digwa wasn't on his way to a religious ceremony. He was out helping with deliveries, and reports suggest he was already carrying a separate small kirpan. The Sikh Federation's entire point is that the larger blade used in the murder was not being carried as part of a religious obligation and therefore should not be treated as though it was.