The exact quote from the Reunion Files was: "Since Sephiroth exists, Aerith must exist. There was no doubt about that one."
As for the context, every quote on that page and the ones around it are from Nojima. The quote is on page 9, and pages 8 through 11 only have quotes from Nojima. Nearly every comment is related to the writing of the script.
For that matter, we've also had several comments from Nomura and Nojima that they knew from the start that Sephiroth would appear, even before they had an explanation for why:
-"I was planning to put Sephiroth in Advent Children since the beginning of the project" (Nomura; pg. 60 of the Reunion Files)
-"We knew that Cloud had to fight Sephiroth in the end" (Nomura; pg. 69 of Reunion Files)
-"During the initial development phase, we didn't really have a very specific reason as to why Sephiroth would come back to see Cloud" (Nojima; pg. 117 of the Reunion Files)
-"We had a difficult time deciding on the reason why Sephiroth would appear" (Nomura;
http://flaregamer.com/b2article.php?p=109&...#ixzz0UoRX0Xok)
So, the context of Nojima's quote then is that, in writing the script, he already knew Sephiroth would be there; and if Sephiroth's going to be there, then Aerith has to be as well.
And another of Nojima's comments (pg. 11 of the Reunion Files) makes it clear that he doesn't think that Sephiroth must always be present in a FFVII story: "These characters have a lot in store for them in the near future, so it's entirely possible that their story will continue to be written. But if someone like Sephiroth isn't included, it could eventually end up as some kind of realistic drama instead (laughs)."