Aerith didn't lead to their creation. It was merely a consequence of readily available Mako poisoned victims.
One thing that is apparent is that the writers are vastly expanding the role, importance and numbers of Sephiroth Copies in FFVII-R.
For one, the pool of Sephiroth Copies no longer is limited to the survivors of the Nibelheim Incident. This new novella confirms this, but it was hinted at way back in Final Fantasy VII: The Kids Are Alright novel.
This is from the book.
The Shinra Company had conducted various experiments on Jenova, the fabled entity that had descended from space. In many cases, the experiments were haphazard procedures that involved injecting Jenova cells or tissue into human bodies by various means. The company had never been aware of the full scope of the experiments, and the details of the work had been lost along with Dr. Hojo, the scientist how had conducted the research - largely without oversight.
Clearly the shambling people the hotel owner had seen were the result of these tests, but there was no way of knowing now what procedures they had undergone. One detail, however, was known to a select few: a phenomenon known as the Reunion. Those implanted with Jenova cells, directly from the source or indirectly from Sephiroth, were drawn to the Northern Cave as if something was calling them home.
Emphasis mine.
This passage coyly alludes to the fact that there were possibly
more Sephiroth Copies than we knew about, and even throws in several curve balls in their creation. Not only were Sephiroth Copies made with Jenova cells, but also literal bits of it's tissue. And it all but confirms that the Sephiroth Copies do have a unique aspect to them. They carry S-Cell type Jenova cells. Meaning, they're Sephiroth derived Jenova Cells.
This is the type of Jenova cells Cloud carries.
This novella now confirms that Hojo wasn't just content with making Sephiroth Copies out of the Nibelheim Incident survivors. He did it to any Mako poisoning victims he could get his hands on.
This would now explain why a Sephiroth Copy was somehow in Midgar in the OG.
