Absence of proof is not proof of absence.
He's not given a number. He could be as low as 80 (though unlikely, given that his sword skills are better than LHC's [who was arguably the best technical swordsman in the novel series], and his internal likely as good or better than YG's). But he also has an untold and only speculative maximum. He is also a "legend," considering he was mentioned in DOMD as someone who was notable as the greatest of his era. I see nothing wrong with putting him where he is. There's no point giving him a number because there is neither proof for or against. Neutral quantities are placed in neutral categories.
The illogical thing to do would be to give him a number without a valid logical argument to support it.
As for his internal, that's because his actual level was never disclosed. We can only guess at it from snippets and what he left behind. He did not leave a manuscript or internal technique, but he did have a training technique, and if we can rely on the story from ROCH, his internal is likely greater than YG's because of his ability to wield the wooden sword and beyond, and it was mentioned while YG was training with the wooden sword that the difference was internal energy.
Also consider that DGQB, by all intents and purposes (and all logical deduction) had far more time and more access to the snake gallbladders and waterfall training method, and it's almost inevitable that his internal (and perhaps substantially so) is greater than YG's.
Not using sword techniques, he wasn't.
A lot of pugilists used their fists. I guess all of them are as good than him.
And Duan Yu's 6MSJ is arguably the greatest thing in the entire JY-universe. If Duan Yu could actually master it, he would probably be at that legendary level. Too bad he couldn't.
Not to mention, even 6MSJ doesn't have a stance that can break theoretically anything. By this kind logic, since DG9J is the only thing that we can say canonically defies the laws of physics (interesting position my left foot) and can be used to incredible effectively WITHOUT internal energy, it's the best.
I bet you aren't satisfied with that, so you shouldn't use the other extreme.