[A reply to a form of this post here challenges me that "scientific inquiry and experimentation" can be part of the mix. I agree. Scholarship in fact can lead to stronger religious beliefs, particularly when it provides a less simplistic view of things. It also can challenge certain faiths. My perspective below is not meant to deny that. In fact, the "provable" comment is not as much a defense of religious belief than a warning of taking it too far.]
Wicca is not a unified movement ... [b]ut Wiccan teachings are for the most part a stew of demonstrably false historical claims. ... The rare Wiccan belief that pans out is that Christmas is an adaptation of a solstice celebration.
A Slate article seems to reject some expressions of a belief system that has many strands. Such criticism (e.g., certain historical claims are demonstrably wrong) is valid, but to the degree it implies all of Wicca share them (even if some are "starting" to see the light), it goes too far. To take one issue referenced in the article, are many Wiccans truly focused on medieval witch burning? And, to the degree the article is correct, it still is wrong to single out Wicca as particularly wrong in this respect. The negative replies are on point.
Various Wiccan beliefs are not really based on "historical claims" as such but beliefs in general. The fact that some Wiccans in part use dubious historical claims makes them fairly typical -- scholars have shown many incidents in the Bible (written down at times centuries after they supposedly happened), for instance, did not really happen.* As an aside, Wicca underlines that belief in a God as such is not what makes "religion," since various religions honor nature or some other cosmology (see, e.g., Buddhism), not a specific "God" figure. Then again, as shown by a locality (blessed by a federal appeals court) that deems Wiccans as ineligible for legislative prayers, such exceptions might not count to some people.
The article implies that Wicca rests on fictional history, when in actuality it appears to rest on various beliefs, some of which some of its members (to some degree wrongly) tries to backdate to times past. I'm no expert, but taking a religion I'm more familiar with (Christianity), I reckon various people who practice it are not that concerned with that aspect of the religion, more so with the rites, morality and various basic beliefs it promotes. Beliefs that are are shared by non-Wiccans, e.g., nature is sacred, reincarnation and that a certain moral path results in true happiness. Again, "religion" is seen in its complexity, specific beliefs not the whole point, but an overall worldview and ways to put it into day to day practice.
Finally, from what I read about Wicca and related religions, some seem to see the "history" cited as fiction here as more akin to myths -- stories that have a symbolic point. In this sense, let's say, the biblical creation stories are "true" in some way, even if we accept they are not "history." As someone once told a little girl:
Virginia, your little friends are wrong. They have been affected by the skepticism of a skeptical age. They do not believe except they see. They think that nothing can be which is not comprehensible by their little minds. All minds, Virginia, whether they be men's or children's, are little. In this great universe of ours, man is a mere insect, an ant, in his intellect as compared with the boundless world about him, as measured by the intelligence capable of grasping the whole of truth and knowledge. ...
Only faith, poetry, love, romance, can push aside that curtain and view and picture the supernal beauty and glory beyond. Is it all real? Ah, Virginia, in all this world there is nothing else real and abiding.
Some members of all religions miss the true meaning of that letter. They focus on the literal, trying to prove what cannot be provable. They act more like historians and scientists than promoters of a way of life. Some, including many Wiccans, avoid that path.
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* The article suggests in some fashion that the false historical claims of Wiccans are harder to deal with because they are newer, thus the people making them can be challenged. But, other religions often make some historical claims which are as open to challenge. The age of said religions give them some degree of authority and some of the things challenged are in the more distant past, but this doesn't stop such criticism.
Again, I see whatever valid message the article sends is not somehow unique to Wicca. Thus, Christians are advised not to rest too much on the exact authority of the texts (a result of disputed translations and somewhat ad hoc choices) found in the gospels or historical events found therein (e.g., the location of Jesus' birth, which is part of a "factual" account that is more important for the overall message it is sends).