“History lesson: The Jedi won the war with Mandalore! These tricks will amount to something, maybe save you from time to time, but they won’t keep you alive in the long run.”
Now, I believe that he had her best interests at heart, and he seemed to be hoping to goad her into performing better with the darksaber, but what he said is vastly uncalled for.
The Jedi and the Republic invaded Mandalore out of fear that the Mandalorians would disturb the peace in 738 BBY, and the fighting ended when the Republic bombed Mandalore so thoroughly that the entire surface was permanently reduced to a barren wasteland. This is still fresh in every Mandalorian’s mind by Sabine’s time, and nearly every Mandalorian still wears the Heart of Beskar (the elongated hexagon in the center of Mandalorian armour), a symbol of remembrance for that event, Dral’han, or the Annihilation.
You can see Fenn Rau isn’t pleased with Kanan mentioning this, and it’s clear Kanan has overstepped his bounds, and you can see how deeply it wounds Sabine that he would bring up such a recent tragedy in Mandalorian history, and emphasize the rift between the Jedi and Mandalorians.