Ah yes I see what you mean, there's no base charge to go up a tier. There's absolutely not by the way, this was confirmed in the FAQ.
I would assume you're right about cup seats, but the way it's worded doesn't suggest that.
Surely there's a charge? Gold is STH with Auto-Cup, and Platinum is Gold with away tickets automatically as well. Both of those have associated costs.
Not quite sure if you won't get your seat as @Bozza says though. I assume it just means Auto-Cup, but it's not clear.