Spangenberger, like many magazine writers, claims evidence exists but fails to provide a credible citation. A true historian would have provided a bibliography specifying the documentation he claims supports his thesis. A politician, on the other hand, simply makes a claim and goes on as if it's supported. It's clear which profession Spangenberger belongs to.
By the way, one only has to read the 5 comments following his editorial (for that's all it is) to see what credibility it lacks. I'd suggest if you're going to cite his opinion as proof of the thesis that you at least be sure the comments that refute it are not immediately available - it makes you appear, well, as if you hadn't done your homework.