Steve Rogers: Big man in a suit of armour. Take that off, what are you?

Tony Stark: Genius, billionaire, playboy, philanthropist.

Tony was being snarky, but he's not wrong about the suit being just an extension. Yes, it's important to his abilities, otherwise he wouldn't have needed it from the beginning in the cave. But it's also not a crutch, as Ironman 3 showed and taught him, and he's trying to show Peter that it begins with the person.

Also keep in mind what he said to Peter in this same scene - when Peter said he just wanted to be like Tony, Tony comes back and says yeah, and I wanted you to be better. Tony knows that Peter truly doesn't need the suit because he is that powerful, it's just once again an extension that enhances those abilities, and if Peter thought it was the suit that made him special he wouldn't grow.