I personally presume, that 'public' means, all NPCs have access to that knowledge if you ask them about it. Regardless if the custom NPC who brought the knowledge into the pool is present in the conversation or not. But they obviously must be enabled into the session.
So, for your blueberry scone incident example, I'd Aldric would probably make something up from his perspective. Every NPC will probably make something up and even conflict with each other, since you specified everyone recalls it differently.
Huh, a 'Lorebook' character specifically with lore is an interesting idea to introduce things into the world that are not about a specific NPC and centralize it all somewhere for managing. I might use this idea, only make the NPC would be a "god of knowledge" so they have an in game reason for being, which is to inject lore into everyone's brains.
Your best bet is to just fill your custom NPC's knowledge set with anything related to them, and mark the parts you want publicly known to other NPCs. As long as it's at least tangentially related to your custom NPC, set it with them. Otherwise, put it into your 'lorebook' NPC. If you want certain knowledge to be intentionally misunderstood, then you'd probably want both perspectives on the same knowledge, making one public and misunderstood, while the other private and only that the NPC knows.