I don't think gp was questioning that, so much as the likelihood of finding more than one fossil from the same event, given all the times we could have found fossils from.

Wouldn’t this single event have a super high chance of generating fossils though giving that a) it wiped out a ton of life in one go and b) it released a huge amount of debris ready to cover things up nicely and preserve them?

Yes. What was incredibly lucky was the fossil stratum being exposed again at a time when we could spot them. There must be megatons more of this stuff, wholly inaccessible.