Italy is more a geographic entity of the peninsula, which in republican times consisted of many other groups other than just the Romans, like the Etruscans, the Samnites, the Umbrians, the Greeks, etc, rather than the modern presumption of a political entity that covers the whole peninsula.

The Roman Empire is called as such because it was the empire of the city of Rome, which later on went and conquered the other peoples of Italy.

However with that said, as pertaining to your question, Ancient Italians would not be an incorrect way to refer to the others co-existing with the Romans before their absorption into the Roman Empire state. But it again, has a specific context of "everyone in Italy" rather than "just the Romans" which is why the two terms are not synonyms.