Not an incident
I’m new here and I’m french so pardon me but I’m not really at ease with english words. Anyway, maybe my memory’s failing but I remembered that in a previous season we see Richard and Ben kill the people living on the island with some leathal gaz or something. So then it’s not the “incident” who killed everyone and then in the present richard didn’t lie he did see kate, Jack and everyone else die. And Dr. Chang is missing a hand in the stations orientation films wich means he lost it in “the incident” and he then survived.
You reference “The Purge” in the first part. “The Incident” is separate. We don’t yet know why Richard said “I saw them die”. I’ll be it has to do with the flash at the end of season 5.