As Sean de Wolski points out in the question that you linked to, a interpolated surface fit has multiple sets of linear fit coefficients, one for each point in the data.
If you want to have just a single set of coefficients for the entire surface fit you will need to use a polynomial fit. For example, if you want to use a 2nd order polynomial in both x and z, set the fittype to 'poly22'. The c.p00, c.p10, c.p01, c.p11, c.p20, c.p02 will contain all of your coefficients.
If this is not what you are looking for could you gives us some more details about what your are wanting.