According to the article, the research was actually there, at least some of it. For the purposes of the book, case studies were changed and divided to be different people. I can actually see why he would do this, too. It's like changing names, and some details, which happens all the time.
The thing is, if someone's case history made it into a book and their friends and associates could tell that it was them, then it could ruin their reputation and they would have a viable position to sue for slander. For books like this, omitting certain details and modifying case studies helps to prevent such things from occurring.
You'd be surprised just how many professionals do this. This isn't the first I've heard about such accusations, nor is Wertham the first to be on the receiving end.
Psychiatrists and psychologists operate under HIPPA, and they need to maintain patient confidentiality. Patients are often used in medical journals and related texts, but their names are omitted. For the purposes of a mass-market text, details concerning a patient's case would be omitted, cases would be changed, etc. Nowadays texts will come with a notice explaining this, but back then, I don't think it was required, as what Wertham was discussing was a relatively new field.