Two of us have answered with the reason. Frontier DEVs didn't take the extra time to remove the notifications for those that don't have Horizons, but all the content is needed for you to interact with other players who have engineer-modded weapons. If you don't have the content on your client you can't render the new weapon effects when they're in the same instance as you. Maybe they will remove the notifications for non-Horizons players at some future date, but you need the content itself to play.
Wait so what does 1.6 do then?