Rinzler himself makes a note on that quote in this article about midichlorians.

[Please note: While we were preparing the text for The Making of Star Wars, Lucas added a note to this passage about midi-chlorians, bringing his original words in line with his later thoughts and the events of the prequel trilogy.]

https://www.starwars.com/news/so-what-the-heck-are-midi-chlorians

So, the original quote was apparently from an archive recording of an interview he had with Carol Titelman in 1977, who was the author of the first Art of Star Wars book, but she also was apparently a member of his staff who was asking him a series of questions that could be used to guide future Star Wars novel/EU material writers.

The quote, as it is in Rinzler’s The Making of Star Wars goes:

”It is said certain creatures are born with a higher awareness of the Force than humans. Their brains are different; they have more midi-chlorians in their cells.”

But, based off what Rinzler said in that article, it probably was:

”It is said that certain creatures are born with a higher awareness of the Force than humans. Their brains are different.”

This info is taken from this Q&A site.
https://scifi.stackexchange.com/questions/7510/when-did-george-lucas-make-up-the-stuff-about-the-sith-and-midichlorians/7527#7527