Tests done since 1933 show that people who talk about their intentions are less likely to make them happen.

Let's suppose this is true.

Announcing your plans to others satisfies your self-identity just enough that you’re less motivated to do the hard work needed.

This does not follow. There is no evidence on which way causality runs. Maybe people who are more likely to succeed, and hence confident of success, don't bother to announce their plans.