Your argument is not valid: it is not true that the quotient of a group which is not a nontrivial direct product is not a nontrivial direct product. For example, the quaternion group $Q_8$ is not a nontrivial direct product (since otherwise it would be abelian), but it has a quotient isomorphic to $C_2 \times C_2$.
But the result you want is true. $L_2(2) \cong S_3$ is not a nontrivial direct product (again, since otherwise it would be abelian), and neither is $L_2(3) \cong A_4$ (its only nontrivial normal subgroup is $C_2 \times C_2$). For all higher values of $q$ it is well-known that $L_2(q)$ is simple, hence has no nontrivial normal subgroups.
I don't actually know how hard it is to prove that these groups are simple. I can prove directly that they're not direct products if $q$ is prime. First, observe that $\text{PSL}_2(\mathbb{F}_q)$ acts double transitively and faithfully on the projective line $\mathbb{P}^1(\mathbb{F}_q)$. It follows that the representation corresponding to this permutation representation decomposes as the direct sum of the trivial representation and an irreducible faithful representation $V$ of dimension $q$. If $\text{PSL}_2(\mathbb{F}_q) \cong G \times H$, then $V \cong V_G \otimes V_H$ where $V_G, V_H$ are irreducible faithful representations of $G, H$.
Since $q$ is prime, it follows WLOG that $\dim V_G = p, \dim V_H = 1$, hence that $H$ is cyclic. Since $H$ is normal, it must be central, but $\text{PSL}_2(\mathbb{F}_q)$ has no center; contradiction.