Firstly RWX couldn't absorb FZ's internal energy, because FZ with the unique character of YJJ could hide his internal energy from RWX, not because his internal energy was more powerful than RWX, although really was more powerful. XXDF had not this principle that you can't absorb internal energy of more powerful opponants. For example most of the people that DY absorbed their internal energy in DGSD had more powerful internal energy than him.
Secondly I think YJJ was the best internal energy in history of wulin since it was said in SPW, Ch. 18 that YJJ was written by the founder of Buddhism and the founder of martial arts, Bodhidharma who Buddhists regard him like an god.
9 Yin manual was written in accordance of taoism principles and was summarized of taoism teachings and included ways for defeating some different martial arts.
Since taoism martial arts also originates from shaolin and since the writer of YJJ was the founder of martial arts, I dout 9 Yin was better than YJJ in regard of internal energy.
But it is right that GJ lived many years in The Condor Tiology and practised 9 yin manual for dacades but LCH just learned YJJ in the end of SPW and the novel ended just when LCH reach his pinnacle of martial arts.