It's Meng's way of seeing it, as shoen with those that observe not seeing the threads, and another guy getting the same enlightenment and not seeing the threads at all.
Well, the first thing is that all the observers could "only sense an aura of essence", which makes clear that the threads he could see weren't actual objects that were physically there.
"As Meng Hao sought Dao enlightenment, the area around him distorted, occasionally expanding, occasionally contracting. Sometimes it would bulge upward and downward, and sometimes it would sweep out to the left or right."
Beyond that, the threads create the space he is inside of (which would obviously be impossible for actual threads to do).
"Within these threads are length, height, breadth, and size. They are limitless, and that is space.... They are flat, but actually...."
And Bai's POV about space in a universe (with time thrown in too).
"The starry sky vanished, leaving behind only… a single point!
It was a singularity of absolutely no length, breadth, or depth!
That singularity existed within the starry sky. It contained all magical laws, all natural laws, and a total of 1,080,000 worlds!
When Bai Xiaochun saw that, he smiled. Waving his finger, he caused the singularity to transform into a line…. It had no breadth or depth, and yet, was infinitely long, with no end.
"I'm missing something, aren't I?" After a moment of thought, he waved his finger again, and the infinitely long line suddenly became infinitely wide!
Because it was infinitely long and wide, it was actually impossible to tell which direction was which. Such descriptions could only exist in one's thoughts.
From a distance, it almost looked like an enormous sheet of paper, spread out through the starry sky. Bai Xiaochun looked at it for a moment, smiled again, and then waved his hand, causing it to grow infinitely deep and high!!
Now, it didn't look like a piece of paper, but rather, an infinitely wide, infinitely long, infinitely tall box!
"Still missing something." Shaking his head, Bai Xiaochun waved his finger, causing the infinitely long, wide, and tall box to ripple and distort. It seemed to age, blurring, as though the passage of time was transforming it into something illusory, into a vortex, into… space!
It contained everything that made up the starry sky for the past tens of thousands of years, their existences stretched out together. It contained all life forms and all living beings, from the moment they were born until they died, from the moment they existed, until they didn't exist. And everything in between. By stepping into that space, one could appear at any moment that had ever existed."