Innocent until proven guilty.
It's jarring, yes, but it's a feature of the adversarial justice system. In criminal cases, the defendant/accused will swear under oath that they are innocent of the crimes of which they are accused, and cast doubt on the accusers where possible. Then, having been found guilty, or having exhausted the appeals process, or having realised that the case against them is too strong and changed their plea to "guilty", the defendant puts up their hands, admits guilt, and nobody holds the lies that they told against them. That's how it works.
Lawyers are, of course, allowed to represent clients that they know are guilty, because that knowledge would be covered by attorney/client privilege. The only people who can get into trouble for lying in court are ones who are not accused.
In fact, you get in worse trouble if you *carry on* maintaining your innocence after the courts have imprisoned you. You are considered to be in denial of your crime, and will not get parole or early release.
So the wording of the old principle can be given a double meaning. In the adversarial system, you have to act innocent until proven guilty.