I see nothing in the article that explains how ionizing radiation may be a part of the, err, energy "food chain" of these small biomes.
Figure 2 of TFA lists the microbial metabolism pathways, notice the absence of "dissociation of H2 molecules by ionizing radiation". The same is true for the long section that expands on it titled "Factors influencing subsurface microbial community composition", where effects outside the metabolism reactions are discussed.
For example, there are several paragraphs that discuss ion and electron sources in those environments:
Microorganisms often make use of the molecules and ions available in the rocks they inhabit, either as electron sources or as sources of limiting minerals (Fig. 2). This includes metal sulfides like pyrite [115, 116], metals such as iron and manganese and their oxides [117], silicate rocks like feldspar that provide a source of phosphorus [118, 119], and gypsum-derived sulfate [68], which are not evenly distributed in all rocks [116]. Profiles of available electron donors in subsurface ecosystems correlate with microbial community composition [120]
Similarly, microbial communities within granite were dependent on mineral inclusions, especially those containing aluminum, silica, and calcium [122]. Another study showed that aquifer fluid type (e.g. gabbro, hyperalkaline peridotite, and alkaline peridotite) was correlated to microbial community composition [123]. Although a single geochemical parameter accounted for the correlation, differing pH, Eh, and availability of carbon and electron acceptors among rock types were predicted to be key factors [123]. As microorganisms use the minerals present in the rock, they chemically transform them.
I don't see any references to dissociation by ionizing radiation. Nor would I expect to see it for the reasons I already pointed out above.
Further on there is a discussion on purely accidental factors like tectonic activity and the mixing of deep groundwaters:
stochastic geological activity may play a role in microbial community structure and succession, with a stronger influence than environmental selection for deep hard rock aquifer systems [126]. The findings suggest that geological activity causing or changing fractures, which leads to the isolation or mixing of fracture fluids and the nutrients and microbial communities within, plays a significant role in microbial community turnover and the establishment of new microbial communities
The only context where radiation comes up is that study is needed to evaluate the impact of deep mining and nuclear waste storage, but the context isn't metabolism.
So, going by this article, ionizing radiation still doesn't seem to play a role in the metabolism of these microorganisms.
