Depends on who you ask and how deep you dig.
Iwamura indicated a while ago that they don't have a good understanding yet how their excess heat exactly is produced although he is hinting at condensed matter nuclear reactions.
From Mizuno I haven't seen any feedback on the theory either.
The only more concrete theory is indicated by Jürg Wyttenbach to whom you likely refer to, or that of Mills.
And then there is NASA with their publication on lattice confinement fusion, which could be applicable here.
I would not exclude the work of Holmlid either although there is no concrete evidence UDH or UDD plays a role in Mizuno's and/or Iwamura's process in my view. Maybe Jürg Wyttenbach can shed a light with his views on this matter.
It boils down to condensed matter that causes (D-D) fusion and/or transmutations, depending of the used elements and the environment (within lattice or at surface and/or impurities/catalysts).
I guess sLRPI didn't make the cut? That has the highest full reads on research gate.