That, and the current canon for Luke’s temple is, I believe, that Ben accidentally blows it up with a big force lightning storm generated by his negative emotions (or Palpatine/Snoke generated a big force lightning storm to blow it up, but making Ben think he did it). It’s not until a little while later that Ben kills his first Jedi intentionally. My point being: it’s not hard to retcon in some survivors, especially compared to Order 66, which was much more well-organised, sudden and widespread.