As the article says, the proposed mechanism is self-annihilation, which would presumably produce energy in the form of perfectly ordinary photons.

That seems wrong. "Dark matter" usually means matter that doesn't interact with photons, not just "matter that is dark because no light is shining on it". But if it emits photons, even in a matter-antimatter reaction, then it couples with photons, and so it can't be dark matter in that sense.

Apparently (eg. [1]) there would be a couple of different pathways available for WIMP annihilation, to a W⁺W⁻ boson pair, or to μ⁺μ⁻ muon pair, or a e⁺e⁻ electron-positron pair, so the immediate annihilation products would be charged non-dark matter particles which would either quickly decay or annihilate into photons or simply shed their energy via normal EM interactions.

[1] P. Salati, 2014. Dark Matter Annihilation in the Universe. https://arxiv.org/abs/1403.4495

Sure, it’s only called dark matter because we find it hard to observe it.