That is an inane comment to make. Also, who said anything about realism?
You are missing the forest for the trees. The real strategic decision comes much earlier, when it is far, far more important. Do I spread my religion early in the hopes of having a slightly higher chance in the future of it spreading on its own, or do I build that Axeman that I desperately need to even stay in the game?
You addressed realism on page 12 =\
The point I was making with the "don't add things based on realism alone" comment is that this is a game. Therefore, because it is a game, the implications on gameplay must be considered before how realistic something is. That goes doubly so for FFH, because it is a fantasy based game, not a historical simulator. I agree that fiction has to be realistic to an extent to be believable, but FFH already doesn't mirror reality based on the fact that there are demons and dragons etc here while there aren't any in real life. So, in conclusion, we should not alter gameplay simply to be more realistic.
As far as the "real" strategic decision: spreading your religion early already increases the chance it will spread on its own, to an extent. If city A, which does not have a particular religion, has a trade route with city B that does, there's an increased chance that the religion will spread to city A.
One of the problems I see with giving a new city a religion for free is that it's a too-easy source of culture. In FFH, there aren't many ways for a city to generate free culture. An obelisk is one, a religion is another. As it stands, both of these have a cost: the obelisk costs hammers to build and a gold each turn. The religion costs a disciple (or, again, you can wait and hope it spreads automatically). If we're going to start giving religions to new cities for free, this dynamic is upset. Potentially, it may also serve to weaken the Creative trait.
So, in conclusion, I still feel giving a religion to a new city for free based on the religious affiliation of the settler is a bad idea.