Of course. The movie is an example of this, relying mainly on tabletop lore, but also bringing in the 80s cartoon characters, which were from a different "expression". The policy basically establishes this as a one-way street, where you can only count on what's presented affirmatively in each expression to be canon for that expression; everything else is unknown.
So, BG III can reference anything it wants to from any D&D source. But that doesn't imply anything else from another canon is "true" there, just what is mentioned. For example, we don't know what BG III's version of the Tomb of Horrors is like, assuming it exists in that canon at all. The same is true for 2024 5e Greyhawk - we will only know what's in the 2024 DMG, everything else is speculative. (Technically we don't even know if Ghosts of Saltmarsh remains canon, since it's not a core rulebook, though I expect they at least won't contradict it.)