Let $G$ be an adjoint Chevalley group. Are all of $G(\mathbb Z)$'s finite-index subgroups congruence subgroups?

I read a theorem that states: When $G$ is the universal Chevalley group and it's not of type $\operatorname{SL}_{2}$ every finite index subgroup of $G(\mathbb Z)$ is a congruence subgroup (in Matsumoto - Sur les sous-groupes arithmétiques des groupes semi-simple déployés).

I've been looking for sources and results on that topic that I can learn and quote form, unfortunately, all I could find was in French.