I don't think that's a good interpretation of the Costco terms. Here's the part that I think you're talking about.
For Executive Member purchases made directly from Costco Travel departing on or after September 1, 2017, a 2% Reward will be earned and applied after travel is completed. Must be an Executive Member when travel starts.
There are two requirements.
First, "Executive Member purchase made directly from Costco travel ..." It wouldn't be an Executive Member purchase unless you are an executive member
prior to the purchase being made.
Second, "Must be an Executive Member when travel starts." This means that you can't book a cruise for next year and then drop your membership level down and expect to receive your reward check from a previous year's Executive Membership. That makes sense because they are just tacking the travel reward onto the reward that you earn for warehouse purchases made during the year that you actually take the trip.
Since they're applying the reward after travel is completed, it's hard to say how the system would work in reality, but I think the intent is that you have to be an Executive Member before you call Costco Travel and make travel arrangements with them all the way through the time that you actually travel.