SpoilerWell, there could have been two flashbacks, one with the lie and one with the truth. Or just the second one.
It just felt unsatisfying to have such a pivotal moment, where the hero's life changes and he rebels against his own family, to be entirely off camera. It's kind of like Black Panther if we never see Klaw or Killmonger die, just someone else saying "Trust me on this."
Though you do remind me of something I do like, which is
Spoiler: More minor spoilersthe movie doesn't do the obvious "You LIED to me, I can never trust you again!" subplot with him and Katie. She largely accepts that yeah, she can understand why he'd keep a past like that to himself and that she probably wouldn't have believed him if he had told her. And I also appreciated that they DON'T hook up at the end and just stay friends.