10:1 odds that this is just a use of the photoelectric effect with a clever dielectric. Based on the press-release description, it is probably not a radiothermal generator (like that used in deep space probes).

1) Have an ionizing radiation source.

2) Get a robust dielectric, with metal plates on both sides of it. Call the plate near the radiation source the cathode and the far one the anode.

3) As the ionizing radiation impacts one side of the cell, the released electron travels only a short distance through the cell. Ideally, the first interaction is with a cathode and the far side an anode, imparting a negative potential to it.

4) A potential difference now exists between the far-side anode and the near-side cathode, from which you can draw current.

Strictly speaking, this is a more like a capacitor which is trickle-charged by the radiation field rather than a battery. There is a design tradeoff between thickness of the metal and dielectric layers in that the initial and final ionizing interaction must be of a (cathode/dielectric -> anode) or (cathode -> dielectric/anode) to get any energy out of it. Ideally, the dielectric has an extremely small cross-section for interaction with whatever the ionizing radiation source is, while the metal plates interact very strongly. However, there will always be some losses due to internal ionization of the dielectric itself. I'm not sure how you would build an effective multi-layer structure, either.

Is this preferable to an RTG? Almost certainly not. The vast majority of the radiation energy in an ionizing photoelectric-effect cell is still released as heat. Very little energy can be captured this way, so it is quite inefficient in terms of energy produced relative to the radiation emitted. You can extract some more electric work by pre-charging the cell such that the freed electron is slowed in part by the electric field gradient, but in practice the bulk of the energy is still lost as heat.

Sea story time! The internal electric field within the dielectric will produce some weird mechanical stresses, too. When my ship's reactor plant went through a refueling overhaul one of the things they replaced was the reactor compartment windows. There are a couple of leaded glass windows that allow Mk. 1 eyeball inspection of the reactor compartment during operation. They are quite thick, electrically insulating, and they build up a large internal electric field along with internal stresses. Standard maintenance was to replace them well before the stress could fracture the glass.

So... could the press release be ...excessively breathless? My guess is a qualified "probably". The physics behind the photoelectric effect are well known and understood. Diamond is special in part because it has a fantastically high dielectric breakdown strength. So, you can extract more work out of the ionized electrons by supporting a very high potential gradient between the anode and cathode, which could be seen as a breakthrough for this type of energy conversion device. BUT it also would enable a new generation of high-density ultra-capacitors. The energy density in a capacitor is limited (in part) by the breakdown strength of the dielectric, since the stored energy is a volume integral of the squared electric field strength. Since ultracaps are technologically much easier to commercialize, and much more investor-friendly than anything involving an ionizing radiation source, but that isn't what the press release points to, I think it is likely to be ... exaggerated.

I could also be wrong, and we'll soon have a new generation of ultracaps!