That's definitely wood decomposed by white rot fungi and should be great for stag beetles, but not so much for rhinoceros beetles if used on its own. I've tried rearing Dynastes granti in just white rot wood before and I still have a larva a little more than six years after its hatching date. It molted into an L3 just a few months ago after I finally put it into fermented substrate when I ran out of the white rot wood I had stored up for it. It didn't die on white rot wood, so it seemed to get just enough nutrition to stay alive and grow very, very slowly.
Remember that most of what you add into your substrate stays there until it's broken down into something with a gaseous state like carbon dioxide, methane, gaseous nitrogen, and ammonia. Between its raw state at the time of addition and the state where it can leave the substrate, it's present as available nutrients and raw materials in one form or another or as the mass of yeast and other microbes. Digestion can process yeast and microbes into nutrients within the gut of the larvae and symbiotic gut fauna can process raw materials to produce amino acids and other nutrients that the larvae absorb.
Remember: digestion is not just our bodies breaking down food into nutrients we can absorb, but also the process of letting our gut microbiota turn raw materials into things we need but cannot produce on our own from things we either do not process or process too inefficiently. Sure, cellulose can be a great source of energy if it's a carbohydrate you can process, but if your gut microbiota can't turn it into the things you need, you won't survive or grow off of it. It's the same idea with rhinoceros beetles--larvae can use it for energy, but it doesn't feed the microbiota they've evolved to keep in their gut in a way that allows them to produce what the larvae need for normal growth and development.