That’s just it, evidently those lost students have nothing to do with the Knights of Ren according to canon, although I think that was RJ’s intentions with that dialogue. I imagine he’d have had the scene go about as we have it in V5 were the Knights of Ren not introduced as a concept in TFA.
The comic book that shows what happened right after the slaughter appears to retcon this and tuck the inconsistencies away.