Finding a reason to carry is really not as difficult as you claim. Here are court documents from a recent self-defense murder trial that contradicts your specific example.

The altercation took place on Thursday night at 9:00PM and the accused had been at a restaurant with his friend on Commercial Drive. His reason for carrying his knife was that he had been using it "earlier in the day while working around the house".

You'll notice the legality of him carrying the knife never even enters the discussion. The focus of the trial was whether or not the use of force was justified since the victim had not yet physically attacked him and did not produce a weapon. (He was found not guilty.)