The idea that they came here to mine ore amuses me so much, it's truly a human exploitation fantasy: as if you could not find all the elements in space. So I think that if they really needed something (...) it probably wasn't metal.
Nah, that's why I said a much more plausible theory is that they would come here to do a
biological survey.
Ore isn't what's special about Earth.
Where ore comes in, would be to assume they needed some resources when they came here. "why would they need resources"? you might ask. But ... resources have mass, and it takes energy to both speed up
and slow down a ship. Even if you use an external launcher, you need to carry fuel to slow the ship down to enter the star-system you're going for. Also, assuming you want to go
back, you need to refuel your ship: you have to launch the ship again, speed it up, then slow it down when you get back to where you came from. And assuming they did use an external launch system to reduce fuel needs of the original ship, then perhaps they'd
also need to construct a new launch system at the destination end, to help them get back. These are just plausible engineering constraints that such a race would face.
Also, once you're in another star system, you can't just ring up and order spare parts, so you'd definitely carry fabrication gear with you, even if the above issues weren't important. So, you probably only want to carry
just enough gear with you so that you can mine and fabricate a new launch system or other needed parts and refuel your spaceship from materials at the destination star system. the fact that ore can be found
everywhere makes this more likely, not less likely.