I can think of no reason for him to interview a seminary rector, nor for them to respond to him, any more than you would have to interview the CEO of General Motors if your Chevy breaks down.
He wasn't writing a book about seminary rectors, though they might turn up in his next book.
He wrote a book about seminarians, who go to seminaries.
A small number of seminarians were his sources, and they made accusations.
If one wanted to maintain journalistic integrity, would not one seek out the rectors of the offending seminaries to get their sides of the stories? Or was he afraid his book wouldn't be sufficiently provacative if it was leavened by their perspectives?