Star Wars Rebels Season 4 revealed what the remaining free Mandalorians on their home planet intended to do, and that took place shortly before the events of Rogue One/A New Hope.
Rebels’ epilogue didn’t reveal what happened to Mandalore, as Sabine hadn’t returned to her home planet and the epilogue was packed with bigger shocks.
If we assume that Werner Herzog’s character is speaking to Pedro Pascal’s Mandalorian here, then it’s safe to say that he’s referring to Mandalore and what happened to it during the original trilogy.