Dictionary definition of confidence:
“full of conviction, confident of success, confident that conditions will improve”
Saying you are confident equates to saying you have belief in a certain outcome i.e. the probability of it happening is high. That belief will be based upon your own ability and/or weighing up the various factors that will impact the outcome and concluding that there will be a positive result.
When you subsequently undermine that positive statement with “what that means I don’t know” then completely goes against the prior statement. You are either confident, or you are not.
The point is Wenger has tried to give the impression of optimism but then instantaneously caveated it and made his statement meaningless by basically saying “fuck knows what will happen m8”.
You being confident in an interview is great, thinking you will “crush it” is a hugely positive statement. You haven’t subsequently undermined your entire confident state of mind with some other mealy mouthed crap about “hmm yeah but who knows what might happen”.
Either be confident and stick to that, or say you don’t know what is going to happen. Simple.