I've heard of shadowing as a way of accent reduction but I'm not convinced it works. It doesn't make any sense that it would. It's just parroting what a native speaker says, but doesn't teach how the differing sounds of a target language are produced physiologically and why they sound different and are unfamiliar to the non-native. All the non-native speaker does is try to find roughly equivalent sounds in his or her native language and use those as substitutes. And that simply doesn't work and that's what causes accents. A speech therapist or linguistics expert who knows your native language and target language and the differing sounds in each is the most qualified to teach accent reduction. That's how movie makers make actors and actresses sound like native speakers for certain movie roles, and much of the time it works well. Lessons in speech are not part of a typical foreign language course, unfortunately.

Language has several different elements to it. Listening, writing, reading, speaking, etc. Shadowing will strengthen listening and pronunciation, but that alone will not give you fluency. It is important to develop other elements of a language as well such as writing and speaking. While you may learn some new words doing shadowing you will need to understand grammar principles to apply them and increase your fluency. One last comment I would make is to be aware that many speakers on TED talks speak with different accents. People from all over the world are speaking in English and some of them mispronounce words so be careful with which speakers you shadow. I wish you the best.

I have no opinion on the answer to your question, but I am responding purely to tell you that you have used the correct word in your question. The one thing I would say about your experience is that it is often difficult for a person to notice their own improvement in speaking, unless they have another person to interact with.