This is simply not true. After reading the Flashback rule above, it is a retcon, but it's a legal one within the rules and in fact encouraged by the rules, and with costs and limitations. That doesn't prevent it from being an effective retcon. And it's certainly not the same as a lore check. A lore check is knowledge only, which is represented by a skill. The flashback gives examples of hiding guns under tables before you got to the game and after finding out that someone tipped off the inspector, going back through a flashback and making it you. That's very different from a simple lore check.