I read it as:
He removed the 3.5" HDD and replaced it with a 2.5" SSD.
Then he then "unfused" it removing the logical volume group in diskutil.
After unfusing the factory drives he left the factory 128GB SSD installed.
Because he has the new 2.5" SSD and the factory 128GB SSD installed and he did not say that he created a fusion drive out of the two new SSD drives it will show up as two separate drives.
Aside from having a single drive it would be pointless and cause unneeded wear on both SSDs by creating a fusion drive out of them. I have never even seen someone create a Fusion drive using two or more SSDs so I don't know if that is even possible. Certainly it is not recommended.
Well, seeing as PCIE SSDs are significantly more speedy than SATA SSDs maybe we'd see even better everyday uses performance? Unnecessary, but would be cool to investigate.