In a brief look at some Zoroastra sites I could find nothing to support the comment about their not accepting converts. It appears they do. I believe the wording in the article was in relation to the Moslems not allowing converts out of Islam. That seems to work better. The tree and the statement above link to ZA sites.
Zoroastrians do not accept converts precisely why their numbers are so small.
There have been some calls for changes in the doctrine, but none have been aggressively pursued.
The fellowship based on birth, may also have begun to protect them from the Islamic assaults which would have portrayed them as usurpers of the followers of Mohammed. Since their community has had to operate as a closed entity, no one in could be equated by the outside to mean no one out either.