Does anyone know much about this author/site? It looks like some sort of Zoroastrian/Zoroastrianism-enthusiast and let's call it 'independent researcher'. There doesn't appear to be much in the way of external references to this voluminous work and some of it seems a little tendentious or at least, at odds with current consensus.
Sites like this are one of the joys of the internet but it's usually easier to figure out the angle.
Can't say much about the linked page but one could read the article[0] about the same topic on Encyclopedia Iranica, which is a better source since it's compiled and edited by scholars in history, linguistics, iranian studies etc. Haven't read them myself but they both mention the epigraphist Grotenfrend at least