"Why commit to a possibility of having to spend Billions to build a mine with a NPV of $1?"

I suppose the logic might be:

We would really like to get someone on the hook to do this exploring for us, but they will expect something in return.

What would be the minimum bar that an explorer would buy into, to do this in a way that gives us the option of backing in only if the situation is economically viable.

If that bar is met, there would be a probablity of substantial additional value, or we can simply decline to back in.

They needed that BFS valuation to get someone to take the exploration risk