"Default"? An over used word on this forum. Anyone stopped carrying knife could claim to be a Sikh or pick from a range of excuses. It is up to the police and courts to come to a practical decision as to whether they are being truthful.
Sure the police might ask some superficial questions to establish whether, prime face, the person is Sikh. But they are never going to probe whether that individual's belief that carrying a knife is a religious requirement is genuine. Are they really going to probe whether that individual always carries the knife, which would be the acid test of the genuineness of that belief. I very much doubt it. If on the face of it the person seems 'credibly' Sikh that's likely to be the end of it.
And the police don't interpret the law, they just apply it. Interpretation would come from the courts and I cannot see how the case of a non-genuine Sikh who once in a while carries a knife more for cultural rather than religious belief reasons would ever come to court to be challenged and therefore the position of the law clarified.