This French vodka maker doesn't quit selling vodka under their own brand. They just also sell some as Kirkland. So don't they have to keep doing a bunch of marketing so that their brand-name vodka keeps selling in all the other stores? How do they save any money on marketing?
I would guess the real reason it works is that it opens up a market of people who wouldn't buy name-brand vodka at (say) $29.99/bottle but will buy Kirkland vodka at $19.99/bottle, yet without cannibalizing their ability to keep selling it (in other stores and in Costco) to people who are currently paying $29.99.
Or another theory is that many Costco buyers aren't vodka experts, so there is some degree of randomness to how they pick vodka. If there are 10 brands of vodka, selling under the Kirkland brand and their own means they are 2 of the 10 brands instead of 1 of the 10, so to the extent that people pick randomly, they get picked twice as often. (It's like buying two raffle tickets.) A similar effect would apply to people who cycle through brands of vodka.