It's unfortunate that this needs to be repeated, but some people still aren't getting the message…
The "futa" in "futanari" literally means "two" or "both". As in sets of genitals. If the character does not also have a vagina, then she's not futanari. It's that simple.
It's not correct just because the writer wrote it that way. It just means the writer doesn't understand the term, either.
And yes, it matters, because having both sets of genitals is part of the fetish for most people who consume this stuff.
Excusing such misuse of the term (e.g., "It's just slang, dude, get over it") is ignorant and borderline racist. You've borrowed the term from another language/culture, you should make SOME effort to use it correctly. If you're really so limited that you need slang to function, use "dickgirl" instead.