Whether it had any real foundation in fact is very uncertain.

Herodotus, who collects with so much pains the strange and unusual customs of the various nations whom he visits, is evidently quite ignorant of any such monstrous practice.
He regards the Magian religion as established in Persia, yet he holds the incestuous marriage of Cambyses with his sister to have been contrary to existing Persian laws.

At the still worst forms of incest of which the Magi and those under their influence are accused, Herodotus does not even glance.

No doubt, if Xanthus Lydus really made the statement which Clemens of Alexandria assigns to him, it is an important piece of evidence, though scarcely sufficient to prove the Magi guilty.
Xanthus was a man of little judgment, apt to relate extravagant tales; and, as a Lydian, he may have been disinclined to cast an aspersion on the religion of his country's oppressors.

The passage in question, however, probably did not come from Xanthus Lydus, but from a much later writer who assumed his name, as has been well shown by a living critic.
The true original author of the accusation against the Magi and their co-religionists seems to have been Ctesias, whose authority is far too weak to establish a charge intrinsically so improbable.