They can't have been from the memoirs, because she wrote the memoirs ten to fifteen years before the Seven Years' War. I suspect it's a letter.

is it possible that she's talking about FW's character (I mean, he seems to have been at least as stubborn as Fritz!) rather than the abuse in particular

I absolutely parsed this as FW's rather impressive and extremely Fritz-like endurance in the face of crippling pain/illness and other hardships, though I suppose it's also possible she saw her abusive childhood as the fire that tempered the steel? But yes, context might make it clear one way or the other.