Yes. Simple oversight is possible. Alternatively, if it is a trust issue, the fact that it's not just "the X.org security team doesn't trust the OpenBSD maint

Yes. Simple oversight is possible. Alternatively, if it is a trust issue, the fact that it's not just "the X.org security team doesn't trust the OpenBSD maintainer team" but "the OpenBSD X maintainer doesn't trust the rest of the OpenBSD maintainer team" adds several extra levels of politics to this, which I suspect is what Theo was alluding to with his "abdication of the duty of care" comment. I assume Matthieu acted (if that is indeed what he did) in his role as a member of the X security team, but I imagine Theo would say that in doing so he failed to act appropriately in his role as a member of the OpenBSD maintenance team. It all makes me quite interested to know what his reasoning was.