"None of them... may not..."

I saw one sentence in a journal paper, which writes in the following manner:

However, C. Duif presented in 2004 a review article [3] which argued that none of the external factors may not be the cause of such sudden changes.

Does it mean that none of the factors causes the changes, and the double negative serves as an emphasis?

Thanks in advance!

I found the sentence in this article. The authors at the time were affiliated with astronomical observatories in Russia and Ukraine. The writing contains some non-idiomatic/ non-standard English, so I wouldn't focus too much on that one sentence, which seems like carelessness.

Examples:

Many scientists remain at their opinion that...

Moreover, the new attitude is growing that ...

The observations were made with the instrument WEB_3, of which a basic description is given in [7].

The fact was very interesting that the torsion balance graph during the eclipse had a tendency to decrease.