Here's what you're missing I think - if the BFS is positive, and it's good enough that TECK would back in regardless of if it has more value or not, then they'll back in. Even using older numbers - most of us think that it's still worth it for TECK to jump in for 75%. So they will. Now the drills add more value to what they're jumping in for.
It's like, TECK takes the option and think the overall asset is worth X, and after the drill results the asset is actually worth X + 5 (arbitrary number). It can only add value which affects the buyout price. It doesn't affect a back-in agreement (or we're assuming it won't) since X alone is definitely worth it for them to back in for 75%.
A buyout is based on a BFS, but not completely reliant on it.