It does count because to reach plat tier you must go through gold tier. It's like saying that the weapon skins are not available for silver tier because you're not technically bronze.
There's a difference though. The Djinn skin is a permanent reward which requires you to place gold 6 times to unlock him. If you reach the plat. rank it doesn't technically count as placing the gold rank or you would also get a bronze and silver tier placement in your tourney stats. You only get the temporary rewards unlocked. I'm pretty sure it works like that, unless proven otherwise. Obviously I can't test it out myself because I already got all permanent rewards.
Temporary rewards: The higher your reached tier, the more rewards will be unlocked.
Permanent rewards: Requires certain amount of specific tier placements.
^ Simple as that
I would seriously be surprised if ND's duo still working on the game thought about that and the Djinn would unlock after 6 plat placements for the simple fact that the tourney stats would show you 0 placements on the gold tier!
It's good to know this at least.
So I checked my tournament stuff today. I reached platinum and have all of the rewards. Under lifetime tournament placings I only have a 1 next to platinum, not bronze, gold and silver. However, I have 1/6 weeks for Djinn under gold rewards. In concluson, it counts as a gold towards Djinn when you hit plat tier.