You don’t even need to believe that much. EVERYONE is a part of the plan of Salvation, it’s just that not everyone participates in the plan and partakes in its fruits. I can plan a wedding, and the bride and groom would be part of the wedding plans, but one or the other might not show.
You have to read the Catechism VERY carefully. Sometimes what it DOESN’T say is as important as what it does. It does NOT say that people who are part of the plan are the same people who are saved.