Einsteins Relativitätstheorie regelt chemische Bindungen in schweren Elementen, wie neue Forschungsergebnisse zeigen

Can someone help me understand why ‚The increased nuclear mass causes orbiting electrons to speed up to a significant fraction of the speed of light, where the rules of Einstein’s theory of relativity are important.‘ if electrons are actually just a probability cloud rather than little balls orbiting in the way we used to draw them?