This is completely made up by TVB. In the novel, Duan Zhengchun was captured by Duan Yanqing while Duan Yu was captured by Madame Wong in two separate events. There wasn't any fighting between the three at that point in the novel at all.
After capturing DZC, DYQ was deathly afraid of accidently bumping into DY on the road, thinking he would surely be doomed if that happened. He reckoned that his only chance would be to use DZC as a hostage, but even then he still didn't think he stood much of a chance. This was after he watched Duan Yu screw up repeatedly at Shaolin, getting punched in the face, kicked to the ground, falling flat on his face, nearly getting stabbed twice etc. So according to DYQ's own opinion, he would lose very badly against DY.
At end of DGSD, DY was able to fight, only his BMSG and 6MSJ was not working properly, the former because he only learn part of it and the latter because JY likes the idea despite saying DY learn from the monks to control his qi after he ascended the throne.
His internal energy was inferior only to XZ and Sweeper and the next closest of XF, XYS and MRB was far away (before he absorbed JMZ he was already beyond them).
My take of a fair fight between DY and DYH, aka without distractions, is likely to be a massacre by DY.
One thing I wish that the SFX people at TVB (or any other production company) had done was to use different SFX for 6 Mak Divine Swords than they did for 1 Yeung Finger Technique. As it was, TVB used the exact same kind of SFX for 6 Mak as they did for plain old 1 Yeung. From the visuals alone, there was no way to tell that Deun Yu's 6 Mak Divine Swords was far more powerful than Deun Yin Hing or Deun Jing Tsun's 1 Yeung Finger Technique. It all looked the same.
It makes it difficult to process how a wimpy guy who doesn't really know how to fight, but has a powerful finger beam would lose to a guy who fights VERY well and has the "same" kind of finger beam.