They should have been able to produce a NPV because they seem to...

They should have been able to produce a NPV because they seem to have carried out a cashflow analysis. Allowance can be made for the early stage assessment with low level of accuracy but it does help the market assessment. The post tax payback period of 3-4 years does give some indication and given a mine life of 3-4 years has also assigned, it seems to indicate a break-even economic case at this stage.
I would expect a longer mine life than 3-4 years when the PFS is produced with higher resources and hopefully reserves defined.
Of course, everyone is hoping for improved REE prices (base case not stated) in order for better economics to be realised but given China's stranglehold, who knows when this might eventuate.
For what has been touted as a low cost simple operation, I am surprised that their upfront capital cost is as high as A$150M. This compares with the optimised DFS estimate from VHM for their more advanced processing plans with a capital cost of A$337M.