What?
Nestorianism was expanding in Central Asia even under the Mongols! When did a religion have practical limits? It had (and still has) adherents from Syria through to China as well as India! Being able to spread across Persia (as it already was) is not impossible. Hell, it was actively interfered with to make it specifically pro-persian by the Persian Emperors rather than pro-roman! It did penetrate the Iranian Plateau, and then the Emperors banished them - it could well catch on again.
It was even taking root in Arabia before Islam came along - and if the spread of Islam by trade is anything to go by, Nestorian Christianity could have spread into Indonesia.
The first Christian State (AFAIK) was an independent Nestorian Armenia!
Considering they had converted some Turkic tribes, it'd be lovely to see an ATL Nestorian Seljuks conquer Persia and make it Nestorian. Wouldn't need no Islam in theory either (even if it would have difficulties).
Hell, considering that Christianity emerged in the Roman Empire with persecution galore, and then made the official religion, I can see a Persian dynasty deposing a Zoroastrian Emperor at the head of a Nestorian people.
There are so many ways Persia could have been Nestorian, or Christian.
Now is that the only way - nope. Personally I'd prefer to see a Sassanid Church that maintained orthodoxy with Constantinople succeed despite Persian efforts - and have that church become the main religion of the Empire, but Nestorian Christianity seems like the most likely shot.
</rant>
Apologies if it sounds like I'm having a go - I just cannot agree with that sentiment when it appears to be false.