Somewhat importantly, no one else claims isolated protons (hydrogen nuclei, which are stable) undergo EC. It's an effect of unstable, radioactive nuclei with to

Somewhat importantly, no one else claims isolated protons (hydrogen nuclei, which are stable) undergo EC. It's an effect of unstable, radioactive nuclei with too many protons and excess energy. Not free protons. In fact, naively that violates mass conservation: a proton+electron combined weigh less than a neutron. With EC, there is excess mass in the unstable nucleus. Here they are claiming something entirely different: the excess energy comes in from other electrons, in a coherent quantum mechanical effect. (An "effective particle", a "heavy electron", takes the place of a physical electron). It's not the same.