I think transforming “it’s possible to think without language” into “language is not a tool for thought” is an overreach. Definitely a lot of our internal voice is post-thought, but crystalizing those thoughts into words can provide footholds for further thought. Some would argue it’s not possible to think through a complex issue without writing:
I’m not sure how you come to that conclusion, even with the internet meme version of the Dunning-Kruger effect. In the meme version, the incompetent think they are most competent, but I don’t think it follows that the most competent would think they are least competent.
I would summarize the actual Dunning-Kruger effect as: people tend to think they are a bit above average, and actual skill factors in only slightly. Worth emphasizing that these results are over groups of people, and individuals have extreme variation.
https://en.wikipedia.org/wiki/Dunning–Kruger_effect