It's absolutely nothing to do with Euro 5 or Euro 6 classification or any loopholes. It's all about the rated emissions/nox value shown in the V5.

Some Euro 5 vehicles are as clean as the oldest Euro 6 vehicles and therefore meet the requirements. Such as in this case and those of Stagecoach.

Euro 6 usually gets referred to by most, as it's the only emissions level that is currently 100% compliant with clean air zones and the only level that can be bought after the zone went live.