I read that to just mean that you can't flood your open groups with lots of cheap Grey card Stormtroopers, even if you physically have the cards. It's a bit ambiguous, and you could read it to exclude the use of, for example, the Hired Guns, but I don't think that's the intent of that rule. They are trying (for balance purposes) to prevent Imperial players spamming too many figures with low threat cost per figure.
This is how I have always interpreted the rule. I am fairly certain it isn't meant to prevent you from using new models, just not more than is offered in the box. I intend on playing this way as well.