I have downloaded Illustris-3-Dark snapshots 135 and 134 and for both of them, potentials for all dm particles are zero. That is to say,
il.snapshot.loadSubset(basePath,134,'dm',['Potential']); yields an array of zeros. For Illustris-3 snapshot 135, dm potential is populated. Are the potentials of the dm particles in the dark matter only simulations provided anywhere or did I simply miss them? The integrity of the data was checked with the method provided.
This does appear to be the case, you're right - I wasn't aware of this before, apologies for any inconvenience!
(There is also no Potential output for Illustris-2-Dark or Illustris-1-Dark, so it is only available for the baryonic runs).