I mean, the sea is hardly the same colour as the sky either (usually) and yet we still call both those blue so it’s not all that different. Though I agree that it seems like starker difference to me I can acknowledge that’s at least partially my own biases.

Yes, if taking it seriously, then those are reasonable conclusions. However, I think you’re just overthinking it, in this context the scenario exists to service the joke rather than the other way around.

The comedic implication is that the giant eagles are natural predators of goblins and, more importantly, hobbits. And might be inclined to a mid-flight snack.