It’s not a joke. It may be a lie — I don’t have enough evidence to know — but the claim is exactly as you said: converting oxygen into hydrogen, not liberating hydrogen from water molecules.
A joke because as a lie it's uproariously funny.
Fission of light elements doesn't produce energy, it requires energy. The protons in bound oxygen have a binding energy of 8 MeV per nucleon. Hydrogen has 0. That means you would need to supply 16 * 8MeV or 128MeV to "split" Oxygen. Producing, in addition to 8 new hydrogens per fission, plus 8 very high-energy (and very dangerous) neutrons.
Even if all the hydrogen produced were deuterium (an isotope of hydrogen with one neutron) you still can't make it work. For deuterons, the binding energy per nucleon is ~1.1 MeV. So in that case you would need to supply an energy of 8 * (8MeV - 1.1MeV) ~ 55 MeV for each fission.
How much energy is that in practical units? About 1.5 Gigawatt-hours to convert 18 grams of water to 18 grams of hydrogen. Not really feasible.