The article is heavily leaning on the paper "The Illusion of Thinking" [1].

It could be boiled down to: in 2025 this paper showed that "thought traces" in the models of the time could sometimes be inaccurate or misleading. Today they still might be, although OpenAI says actually they are accurate for their modern models, (based on internal research, rather than published research).

[1] https://arxiv.org/abs/2506.06941