It indicates that 4x is square-free, but that doesn't agree with its documentation, which states
Returns whether the polynomial poly is square-free. A non-zero polynomial is defined to be square-free if it has no non-unit square factors. We also define the zero polynomial to be square-free.
See also Nemocas/Nemo.jl#1742 and Nemocas/Nemo.jl#1311