Sounds dangerous to decide the winning number in advance. Makes it easier for a vendor or someone else to cheat.
But I suppose they wanted short numbers.
It’s almost like when in a high stress situation you tell someone “don’t press the red button” or on a tight rope “hey don’t look down and lose your balance”
Ultimately they didn't test. Or confirm the vendor's test. It's all Pepsi's fault for a mission critical bug.
Random audits (with expensive penalties) could have detected whether the manufacturers had complied with this, without making it too obvious to contestants that the winning number would be a multiple of 15.
The winning bottles could then be manufactured to contain some specific multiple of 15, while still allowing the organisers some freedom to choose which one.
Though, making it non-forgeable might be trickier.
You need a system where no one can identify a winner until after the caps have been printed, and the soda has been bottled and shipped.