The paper mentioned and the formula cited by Itronneberg still refer to "raw" (non-central, "at the origin") moments. To verify this, take n=3: you get $\kappa_3 = \theta_3 - \kappa_1\theta_2$. (EDIT: fixed according to comment by Gâteau-Gallois) Hence, $\theta_n$ clearly denotes raw moments: the cumulant and the central moment should coincide for $n=3$.
Indeed, the paper quoted doesn't have any reference to central moments.
Another paper, however (which also mentions the former paper), does: Relationships Between Central Moments and Cumulants, with Formulae for the Central Moments of Gamma Distributions. And it provides the following formula (eq. (2.2)):
$$
\kappa_r = \mu_r - \sum_{j=1}^{r-2} {r-1 \choose j} \mu_j \kappa_{r-j} \qquad r \geq 2.
$$
Indeed, if you set $r=2$, you get that the summation is empty, and $\kappa_2 = \mu_2$.