They never said that. They said that their ability to use the Force had dimisnished, and they never directly linked it to the Sith. They said the Sith knew about it, but they never said the Sith caused it or even implied that they did. Basically it could be the natural way of the Force to gravitate toawards one side for a period of time and then gravitate towards another side for another period of time.On the topic of the Force and Jedi/Dark Jedi powers Yoda and Mace did say that the Jedi's ability to use the Force has been diminished (by the Sith), and it was implied by those two that it was great enough that it was so severe that non-Jedi can seriously hurt the Jedi. Just note the fact that the closest that a non-Jedi has ever done harm to a Jedi in film canon (previous to AOTC) was one of Jabba's men hitting the back of Luke's prothetic hand.
And it was never implied that their actual power in the Force had been drastically weakened. It was implied that their clarivoyance and awarness were weakened somewhat, but that's it.
Of course if you can provide me with a quote that disaproves me on any of this I'll gladly concede my points.
Alternate quote: When it comes to absolute canon, the real story of Star Wars, you must turn to the films themselves - and only the films.His Divine Shadow
Canon policy > you
ICS and the VD is canon because canon policy dictates that it's so, it's the same with the novellizations and scripts, because they all work directly in conjuction with the movie and therefore the official story of SW.
"Canon refers to an authoritative list of books that the Lucas Licensing editors consider an authentic part of the official Star Wars history"
This includes the novellizations, Radio Dramatizations, scripts and any books based directly on the movie in question, wich means the ICS and Visual Dictionary.
The quote you are bringing up, seems to refer to the official Star Wars universe. That's the key word, official. As in the universe of the EU. Which only means that there are books within the EU that are of a higher status than the others.