NTSB's M.O. has always been that there is never just one cause. A human mistake that costs lives is never that simple. There is a system that trained the person, a set of incentives that put the person into that place, a set of safeguards that should have existed to prevent the mistake from causing life loss, and a regulatory framework to occasionally verify all of the above. I would expect that "the controller made a mistake" would be ~one paragraph in a 100-page report.