The idea is that it gets mostly blocked by the combo of the layers of clothes and skin so that kind of optimizes you not absorbing dose for that contrived scenario. If you swap beta with alpha then the beta will be doing more damage because it can penetrate the skin while the alpha is still entirely blocked. If you swap the beta with the gamma then the beta will be depositing basically all of the energy inside you and the gamma will not really be shielded at all from being in the pocket so that’s also more dose. If you have the neutron source swapped with the beta then although the beta is no longer a problem the neutrons will be penetrating you and moderating throughout your body which is bad. So beta in the pocket is the optimized option for that scenario even if it’s not necessarily where you would want to keep it in real life… similarly don’t actually go around eating gamma sources lol.