Ann: Appendix 4D and Half Year Financial Report, page-84

You are quite correct. WHC's net debt at 31/12/21 was A$403m and they are forecasting net debt to be zero in March 2022 (WHC actually say net cash). Hence WHC's FCF for FY2022 Q3 will be around $400m and if Thermal Coal prices remain at current levels it would be reasonable to expect the same level of FCF in FY2022 Q4 which would pay for the entire Share Buyback. So it would be possible for WHC to be Net Cash or slightly Net Debt on 30/6/22.
This is a very smart move by WHC because they believe that their share price is too low based on its current production forecast and it is the perfect time to buyback shares. The buyback of 100m shares commencing on 7/3/22 should support the share price and but a smile on the face of shareholders and management alike which can only be a good thing.
Todays SP moves are either a misinterpretation of the ASX announcements by the CTAs or the CTA's having some unpleasant fun at shareholder expense. Hopefully they take a cruise soon.