Why can the elements of the dual space of $\ell^\infty(\mathbb N)$ be represented as sums of elements of $\ell^1(\mathbb N)$ and Null$(c_0)$?

<hr:

EDIT: As [confirmed in the comments](https://mathoverflow.net/posts/comments/148666), the OP intended to ask about this sentence
"$f\in\ell_\infty^*$ is the sum of an element of $\ell_1$ and an element null on $c_0$" from the paper D. H. Fremlin and M. Talagrand: A Gaussian Measure on $l^\infty$ https://www.jstor.org/stable/2243023 (Which is different claim from what was in the original version of the question.)