Game time is applied to an account, not a platform. It wouldn't make sense for a time card to only apply to one platform when you can play with a single account on multiple platforms. It's just the category they've stuck the cards in on that site. Also, Product Details section on that page:
Product Details:

60 day Pre-paid Time card for Final Fantasy XIV: A Realm Reborn

Compatible to work with PS4, PS3 & PC platforms