Like previous post, it has nothing to do with the quality of metallurgy. In addition the Russians are also using Titanium for their hulls.But you are putting the cart in front of the horse and not the other way around. In order to put in all those anti vibration raft and the associated mechanism and damper, you need much bigger diameter than 10 or 11 m hull. That is design requirement there is no bargaining in it.
Looking at the cross section of the those sub It is cramp as hell there is no way you can fit in those anti vibration raft in 10 or 11 m diameter hull
Whether they design double or single hull it is irrelevant It all depend on the strength of Chinese metallurgy and how advanced they are in producing high strength steel . China design the sub with double hull following Russian practice But I think they can drop that practice now that their steel industry can produced high strength steel
Rail gauge should not be the limiting choice since they can support the carriage with large overhang
People are looking at it in the wrong way. It is the machinery that is used to press the these extremely strong metal plates into complex hydrodynamic shapes. Compared to the US, the Soviet Union is behind on this.
So what do you do? You have an extremely strong pressure hull but because of your machinery, you can only press them to crude basic geometric shapes, like a cylinder. But your exterior hydrodynamic hull, which has thinner plates, can be pressed into any form you want, allowing for more complex geometry.
There are still limitations in pressing pressure hull grade plating into hydrodynamic forms, which is why US submarines tend to have simpler, nearly generic designs. Even then, the pressure hull stops where the nose is, and the bow of the submarine where the sonar sphere is, is much thinner plate that can be formed into a more hydrodynamic curved surface, assuming they are not using polycarbonate.
The Soviet Union also has another factor requirement for fast submarines to chase down US carrier groups, which require highly streamlined submarines, which in turn, require teardrop shapes with more complex surface geometries.
Nowadays, China's machine tools have progressed leaps and bounds from the time of the Han submarine. Remember the one of a kind press used to roll press the 055's hull plates? China can certainly afford to drop the practice of building double hull submarines unless you find some other benefits of using double or hybrid hull submarines, regardless of the state of your machine press technology, like Japan does with the Soryu class.