The source is just all made up, unfortunately. The paragraph starting “The annihilation of a proton with an antiproton is shown in Fig. 2b” and the figure itself bear no resemblance to reality. It looks like the author of that write-up and the site in general is a science fiction enthusiast who has written articles to provide some scientific foundation for those sharing his enthusiasm. He’s been more prolific than careful, it appears.
Proton-antiproton annihilation is a strong-force process, so his diagram’s fundamental emission of gammas, muons, and neutrinos is already problematic. In reality, you get a random draw from a messy menu of mostly mesons, subject to conservation laws and kinematic shaping. Pions, being the lightest meson, show up most readily. And in almost all cases, the “released” energy is mostly kinetic and will be ultimately deposited, even without the eventual recovery of some of the mass-energy as well.
(Aside: In writing this reply I realize that I made a mistake en route to my ~85% number above. Replace that with ~92% efficiency [i.e., non-neutrino energy].)