Mattew is confused and thinks the two Apocalyptic events are the same when they are not:
The first event was only shown to occur when the present Soul King at the time died. Yhwach's father was finally killed by Ichigo. Mimihagi was absorbed by Yhwach and thus the collapse of the world began anew until Yhwach absorbs his father's corpse and took his place. When Yhwach died the worlds didn't collapse or shake in the manga, as explain the sequel * novel his reiatsu and corpse were still holding the worlds together.
- World of the Living (Earth)
- Soul Society (The Afterlife)
- Hueco Mundo (The World of the Demons/Hollows)
- Muken (A pocket dimension void that stretches for Infinity)
- Dangai (Is an isolated space which is surrounded by numerous stacked-up layers of time's current, resulting in it being a place where the density of time within it to be extremely high when compared to other exterior dimensions where other dimensions reside in.)
The second event was Yhwach's power that he was going to destroy:
- World of the Living (Earth)
- Soul Society (The Afterlife)
- Hueco Mundo (The World of the Demons/Hollows)
And create a new world after doing that. With time passing in the manga, he ditched the idea of destroying Hueco Mundo and left it for last, he only focuses on destroying Earth and Soul Society first in the end with his own power reiatsu which begins to climb up to the surface from below Soul Society.
Now these 2 events are completely different base on:
Contradictions from the lore:
The world was not shaking apart when Yhwach was trying to destroy them. The current Soul King was alive. (Yhwach) From the novel, even when Ichigo kill him, his reiatsu was seal with his corpse, which was holding the worlds together and they didn't shake or collapse either.
The collapse of the balance of souls was only shown to happen only when the Soul King dies. (There's no canon proof that the current Soul King can do it willingly. Anyone who says that Yhwach's let's go of the balance is making headcanon accusations.)
Yhwach was using his own reiatsu and power. (Ichigo calls it that when he absorb it in the previous battle.)
Ichigo isn't part of the balance of souls, yet he was getting hurt by Yhwach's last attack.
During the last half of the arc, it was stated a lot of times that Yhwach was going to destroy the three worlds with his own power, and create a new one.
The existence prior to the Soul King was a state of "nothingness", there was no world waiting for Yhwach if he let go of the balance of souls.
Yhwach planned to make his own world by fusing live and death.
Yhwach already proof to have the power to move spiritual matter around, (not convert it into energy like Quincy normally do) and create stuff on a smaller scale without any effort. By small scale, I mean six cities in the sky connected with bridges. ...
Not fuse them literately. Destroy them, and then make a new one from their remains, according to the manga statements. That was the threat from the last arc that made three different races joined forces to stop his plans.
But he was killed before doing it.
Here is the scan that shows Soul Society and Earth dimension been the same size: