It is true that the connection between Technical communication, usability & UX dates back to the ’70s. I have met usability professionals from cognitive science backgrounds and I have always thought how a usability professional from technical communication is different from one from a cognitive science background.
From a personal experience, usability experts all share the desire of enhancing the user experience and making things more usable but their backgrounds to a degree dictate their approaches to usability.
Having a usability professional colleague from a cognitive science background, I know that he is well vast in a controlled laboratory research environment thereby traditionally emphasizing the human brain and how users process their information. He has experience in computers and he focuses on mobile devices, wearables, video game consoles. I realised he gravitates toward quantitative data approaches like mouse clicks, error rates, eye tracking and all that stuff that can be statistically quantifiable.
On the other hand, usability expert from technical communication have experience in user centred approaches, cultural, social & human factors in the use of technology & ethnography which generally means that they are vast in quantitative approaches as well audience analysis, content analysis, visual analysis etc. So I for example, from a technical communication background discovered that I tend to focus a lot more on the audience or user and approach usability testing from a user perspective and use the think aloud testing and heuristic testing more.
So in a nutshell, they complement each other. Recently companies such as Microsoft have been have been recruiting usability professionals from technical communication, ethnography to complement their cognitive science professionals. So a usability expert from a technical communication background is not inferior and no one background is superior either, it all depends on the target users, their experience with technology, the context and the test object itself.