This has always been a spot where I've hated the wording, but I just looked at the ZMG rules, and I think they did a better job of it.
First up, the bullet point on page 107 of the CAR:
At game end, all uncompleted castles score no points and their followers are removed from the board. A castle gets no points for farms.
This bullet point is talking all about how many points the follower on the castle gets -- in this case none, since the castle is incomplete. The second sentence is simply a clarification that the follower on the castle does not get any points for a "completed" farm in its area, because castles never get points
from farms. (The phrase "for farms" is confusing, but ZMG does a better job in their wording and by referring to the castle area as the fief.)
As far as footnote 340, that was just highlighting a mistake that RGG made in one place. All of the rules everywhere else (HiG, ZMG, elsewhere by RGG) state that the owner of a farm gets 4 points for an included castle, with no mention of the completion status of that castle. Thus, whoever put the little scoring help guide together in RGG Big Box 3 dropped the ball for that one bit.