There are different personality types. Most people choose their beliefs to avoid ridicule and undesirable social consequences. Some people are concerned about the truth, and seek it even at the cost of unpleasant social consequences.
The latter type of person is who this backchannel was created to communicate with.
By starting off with a prediction that was false, Q made 100% of the people who choose their beliefs based on the fear of ridicule stop paying attention and go away.
The subsequent valid Q proofs were intelligible to truth seekers, but ridicule avoiders couldn't understand that they proved that Q was working with POTUS. For them, the single incorrect prediction meant that there was a valid reason for ridicule, which nullified everything else.