Apparently not no. Thats is the point in investigation. The fact remains that an independent witness is not required.
However, is that the act which led to the caution? I think not, the act of throwing a knife and it missing it's intended target landing near someone else, does not support a charge of assault. No intentional application of force, or threat involved. And more so, as the person cautioned wasn't the person who threw it!
Hence why I pointed out that the catapult part is probably the cause of the outcome. It does fit the evidence required for common assault, the knife part does not.