It's about antibodies, not spike proteins. Antibodies is the stuff that binds to spike proteins. That has two effects: Antibodies can block the part of the spike proteins that binds with cells, then they are "neutralizing" antibodies - the best of the best. But even the antibodies that bind elsewhere are valuable: They mark the spike protein so that immune system cells can find and destroy them (they act as target painters). Natural infection creates more neutralizing antibodies than vaccination. Monoclonal antibodies are neutralizing.