How else would you define there to be only two winning numbers then? If you define the winning number afterwards, there’s no way telling how many numbers of those would’ve been in circulation.
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.