The original post in the thread doesn't say that derivative works can't be copyrighted. It only says you can't legally make and sell a derivative work without permission from the original copyright holder. But if you do make a derivative work—because, for instance, you do have that permission—, you absolutely can copyright the derivative work. And if you were using the OGL, at least prior to this OGL 1.1 debacle, that license gave you that permission.
Tazawa is absolutely correct. Derivative works can be copyrighted—at least the new material in them can be. Heck, the OGL (1.0) itself tells you to add your copyright notice to Section 15. Why would you add your copyright notice, if you couldn't copyright the work?