In other countries, like Canada, they also ran with a disclaimer.
It wasn't obviously a joke. At the time the expenditure on actually providing it would have been ground breaking, but not too many years later marketing spends were high enough that it would have been a reasonable cost of business (and a phenomenal promotion to actually do it). There are even insurance programs now that focus on fringe outcomes in marketing campaigns, specifically so you can have unlikely to be claimed grand prizes at a managed level of risk.
In fact, I believe Pepsi's loss on the jet would have been pretty similar to the Taco bell 2001 MIR splashdown promotion (where they offered free tacos to every American if MIR hit their target).
