Yes but when you were making up to $100 per tonne in margin and...

Yes but when you were making up to $100 per tonne in margin and increasing volume lowers costs by what $10 maybe ?, now the excess supply cause market price to reduce to $70 (maybe lower going forward) i.e less than the margin they were getting previously

Remember expanding capacity costs money too.
Going forward if RIO and BHP doubled production what would could we expect IO price to be
and it must cost a lot to double production - especially if IO price really crashes and some of the capacity is left idle (hopefully that scenario is unlikely but don't rule it out)