That may not be the major reason for the herem, but it certainly is proof that it is correct.
I mean once you see this, there ought to be no question that you are dealing with a cult of idolatry , not Torah.
If you consider that herem to be valid, the result would be vast. For the law of herem is not to have thing to do with the person or people. They can not learn or teach Torah. [Though I consider Rav Nahmna is be OK as he was outside of the strict category that the herem was on.
Furthermore, it is a positive commandment to destroy idolatry and everything that was made for it. So teh fact that the signature of the Gra concerning this issue, means that people are not destroying idolatry when they are able to do so.
[In Israel, it is a positive command to search and destroy. Outside of Israel, only if one encounters it.]
