That may be true but changing the department's name can only be done with an act of congress, which has not been done yet. Thus, the name is still officially and legally Dept of Defense.
Just because a name is more accurate doesn't mean that it's its new name. Otherwise we wouldn't be the United States of America (we are literally not united bc Hawaii and Alaska are not contiguous, and we are figuratively not united because... Well, you know)
The person you're responding to probably hasn't read the book and is just parroting the word. That's kinda where we're at right now in society. I see the comments by malfist and abustamam are similar. No idea what newspeak means, just parroting and saying "that's not its name".
The problem will get worse as we have a generation raised by LLMs.