This summary is not the rule. The rule text controls.
If someone thinks a criminal case is being tried in the wrong parish, they need to raise that problem before the trial starts. This is done by filing a motion called a "motion to quash." A judge alone decides this issue, not a jury.
Venue (the question of which parish has the right to handle the case) is not something the prosecution has to prove during the actual trial in front of a jury. Instead, it's treated as a separate legal question that gets settled before trial begins. The prosecution still has to prove venue is correct, but the standard is lower than what's needed at trial. They only need to show it's "more likely than not" that the case belongs in that parish, and the judge makes that call ahead of time.
This means defendants can't wait until trial to argue about venue. If they think the case is in the wrong location, they have to speak up early and let the judge sort it out before the trial gets underway.