New evidence supporting the existence of the hypothetic X17 particle

It seems strange at first that a particle with a mass of 17 MeV (much less than a proton) would have gone unnoticed, but the particle they describe is protophobic, that is, it doesn't couple to protons, so it wouldn't show up in p-p accelerator experiments.

In nucleon interactions you would have neutrons around which would interact with protophobic particles.

What I still don't get is why the X17 doesn't show up in e-e colliders. (An X boson couples to both leptons and quarks, which is why it can contribute to proton decay)

A protophobic X boson would not contribute to proton decay which could explain why proton decay is not observed.