The concept of doublethink does assume that truth is binary in nature.
In real life, it ain't so simple. We have, as Asmiov wrote, "degrees of wrong". However, as he pointed out later, whilst chatting to John: "...when people thought the earth was flat, they were wrong. When people thought the earth was spherical, they were wrong. But if you think that thinking the earth is spherical is just as wrong as thinking the earth is flat, then your view is wronger than both of them put together."
This is the essence of science, I think. We get approximations to how the world works, but we always know that our knowledge base is incomplete.
I'm using doublethink, if the truth of a particular subject matter is binary;
I'm not using doublethink, if the truth of a particular subject matter isn't binary.