[The Only Real Estel]

Boromir was alone with Frodo when He talked this way

Although Sam's speech to Faramir in Osgiliath (in TTT) would indicate that Frodo had told him of his encounter with Boromir. Still, Boromir never used those words, so you're right that Sam couldn't have been speaking of Boromir when he was talking to Frodo.