"Decay" is an unfortunate historical word for what are essentially bidirectional pathways between groups of particles. It usually just means "transform". We know a proton can transform into a neutron, positron, and (electron) neutrino in "beta plus decay" (and the reverse can also happen, and all sorts of other things). This is all the answerer means when they say "decay". When this transformation occurs, all conservation laws must hold; in particular, charge conservation. Therefore charge(neutron) + charge(positron) + charge(neutrino) == charge(proton), and we know charge(neutron) and charge(neutrino) are zero, so charge(positron) == charge(proton). I suppose it's possible we don't have a full picture of beta-plus decay, and there's some nearly undetectable fourth particle carrying off a tiny bit of charge, but my understanding is that a lot of the rest of our understanding of particle physics would have to be wrong for this to be the case.

This is not the same as "spontaneous proton decay", which has not been observed.