The Master Chief has been paying quiet attention this whole time, learning. Originally it was Toxin that caught his attention, symbiotes not being particularly normal on his side of the fence, and so he's been listening in on snatches of this conversation. It's a good distraction from the goo. While it doesn't quite trigger his gag reflex, it's much easier to down when you're not thinking about it.

"It's a planet," he puts in. "Birthplace of the human race. Sol system, third rock from the sun." It doesn't seem to be classified information here, particularly due to the lack of Covenant, and it's not his universe. Especially if there are people that don't know about Earth.