the medieval ruling class in Britain essentially all killed each other and have no descendants today

I'm one of their descendants.

I rather assumed that every Anglo-Saxon was (excluding the royal family through Charles, whose ancestry is German, but including Diana Spencer and her children), and that I only knew how because I had wealthy ancestors who kept track. But even if that's not so, they don't have no descendants.

ETA: On second thought, perhaps the scope of ‘essentially’ was meant to extend to the end of the sentence.