The participant in the test is asked to randomly press either button A or B. The cognitive/mental experience of the participant is the following sequence of operations that seem to happen sequentially in time: 1. The participants is asked which button to push. 2. the participant ponders his decision. 3. the participant pushes the chosen button.
But what this test shows is that while the participants internal monologue is still 'deciding' whether to press A or B in step 2. The software is already able to accurately predict the answer at step 1.
So this indicates that even though your internal monologue might still be going "hmm should I pick A or B?", the decision, where your internal monologue eventually will end up, has already been made.
This is what the clickbait headline refers to; even though it feels like we humans are able to dictate and direct our thoughts, it seems that it doesn't entirely works that way.