Look at that passage, it says "Nephilim", not "Demons". They are not the same thing. Although the info above is outdated, what it meant is that not all "Nephilim" are Demons. Only those under Satan are called Demons, even if they are of the same race or type, because "Demon" is a title, not a species or race of beings.
There is another passage that says "not all Demons are of Satan" in the summoning page in JoS but you have to read it context to understand what it means.
What it means is this: Not all the beings that the enemy calls "Demons" are of Satan. Because non-Satanists believe "Demon" means "an evil being", not a Gentile God.
There's much misinformation in mainstream sources where they talk about Demons and they tend to include every being they believe to be "evil" as a Demon. So they usually mix up enemy angels, greys and other nefarious beings together with some of our Gods and call them all "Demons" because they think "Demon" means an "evil being". So when newbies read these enemy books and then read JoS where it says Demons are not evil they become confused and try to summon some enemy angelic being that they read in some other source where it claims that it's a "Demon" and run into disaster.
So the author of JoS felt necessary to include that warning, using the word "Demon" as the enemy uses it, not as we use it because she was addressing newbies.