“Figment” is defined as “a fabrication, fantasy, invention; something fictitious.”
That’s what I expected, but I realized I had never heard the word without the expository “of imagination.” That made me think that perhaps it meant something different.
It turns out the phrase is just redundant.
On a slightly related tangent: one of my favorite movies, the 1983 George Lucas film Twice Upon A Time, contains characters known as The Fig Men of Imagination. You should go rent that one. You’ll love it.