They have to submit a mine closure plan as part of the MMP...

They have to submit a mine closure plan as part of the MMP though to get approval to mine in the first place. They cant just be like "yeah we'll probably sort it out in the next 20 years".
There is a history in Australia of companies doing similar then being disappearing and leaving the state with a huge cleanup bill.

IMO management are between a rock and a hard place as they wont yet have a clear idea of how the project as a whole (not just Grants) will fit together.

Obviously there are efficiencies that can be realised which would increase economic viability but they also need to start mining ASAP to generate revenue and they likely dont and wont have all of this info when they want to start mining.