I think that the advice is that just because you know you are innocent that is not enough, they will still assume you are guilty (regardless of innocent until p

I think that the advice is that just because you know you are innocent that is not enough, they will still assume you are guilty (regardless of innocent until proven guilty) and that you need to come at it from the point of how can I change their mind from me being guilty to me being innocent. Imagine you are innocent and you naively believe they can't convict you because you know you are innocent. There is no device to test for innocence so in the end it is not that hard or impossible to end up in jail even when you are innocent.