Imagine they release 100 tons / year.
The world supply of platinum has just gone from 100 to 200 tons / year.
Or they release 200 tons, and the world supply goes up to 300 tons / year.
At some point really low they'll make platinum as common as dirt and worth about as much.
* exaggeration to make point. Point stands.
You cannot value the asteroid at weight * current_price_per_ton.
Because the earth is surviving on 100 tons of platinum / year, and the current_price_per_ton is based on demand to supply.
If you rocket back with a whole 100 tons in the first year, you dramatically affect supply. The price drops; platinum suddenly becomes more affordable for more things, demand may increase and so on.
But we can all see that if there were mile cube of platinum delivered to earth, it wouldn't actually be worth $1500/ounce any more.