If you believe that only members of your immediate religious group are going to heaven,
That's not it.
It doesn't have anything to do with whether or not people who were not part of the Orthodox Church in this life can eventually wind up in Heaven or not; I am fairly confident they can.
On the other hand, I understand holiness and redemption as particularly a phenomenon of the Church, and furthermore that Sainthood is a matter of what role a person played in the Church's
salvation history. If someone wasn't even part of the Church obviously they cannot play such a role.
On top of that, I think for someone to be rendered a Saint in this life they essentially need to have access to the means of redemption. That's really the only way anyone can be rendered free from sin.
His writings make you stop and think to yourself how insightful and what an intellect this man was even after 13 centuries.
One can have charismatic ability and have a divine vision without necessarily being holy.
He was a holy man,
Also, one can be quite religiously pious without necessarily being holy.
He didn't write and argue for or against the "heresies" of his time. I really don't see a problem here...
It doesn't particularly matter if he wrote in favor of Nestorianism or not if he was part of the Nestorian church because the Nestorian church is devoid of the means of redemption.