Elena is not as lost with these concepts as one might expect, considering she was born in a much different time period. But the supernatural world within that world, the one she was dragged into - she understands some of what he speaks. Understands the waging of war, and the power plays deployed in search of submission. She doesn't need to know what dragonglass is, exactly, to appreciate the difficult position he was in.

He would have been expected to protect the interests of the realm he was chosen to lead. But how could he have done that if the largest threat of all was not dealt with? She gets the prioritization, even if she's never had to make such large-scale decisions.

"You did what was necessary."

Not well-favored, perhaps, but necessary.