"Have I missed something in my reasoning -" Yes, I think so.

Just take a second: Was Voisey's Bay bought to keep it's resource from the market?
Obviously not.
Yes if big enough it would depress nickel prices but again consider, by the time it is built up and producing in a volume that it should happen the other mines will be partly if not some completely depleted. No, the market for more nickel is and will be there. World population is growing and so is the need for more steel products containing nickel.
Is is still a nickel play, chromium has always been a smaller brother usage wise and no reason for that to change.