That's not a rhetorical question. They have similar nutrients to regular food, but we've known for a long time that nutrients weren't simply absorbed. So, really, the first thing to ask is "what is digestion"?
The Mouth. 600 bacteria, a vast number of enzymes, pH is alkaline. This is the first step, for obvious reasons. The enzymes break down certain chains, but obviously they don't get out a chemistry kit and notebook to do this. Any bond of the right type will be attacked. The bacteria are even more complex. They need certain chemicals to survive and will excrete certain chemicals as byproducts. Humans evolved to use these bacteria, some are older than our species, and we need the work they do. The good bacteria are also front-line defence against bad bacteria. Some of the good bacteria will produce toxins that will kill the bacteria you don't want. Others simply don't provide space. A well-fed mouth is essential for good health.
The stomach is rather boring. The only interesting detail is that it absorbs alcohol.
The gut has 1200+ species of bacteria. These control your autoimmune system (diet and environment thus determine allergies to a large extent), your brain (always a good idea not to upset organisms that can nuke your brain) and your general health. They also do virtually all the remaining digesting. Really, the big difference between humans and, say, sheep is that our second stomach became one with our gut. The same principles remain at work. Bacteria break things down, obtain energy and push the discards into the bloodstream.
Our understanding of gut microbes is very limited, I'm involved in the research on them. What is certain is that they are absolutely essential. You need the right amount of each species and subspecies. Red wine results in a massive boost in genetic diversity, a result you don't see from garlic or grapes. This may be linked to the French Paradox, as other cultures that indulge in red wine do experience greater longevity without the increases in cancer linked to alcohol.
What else do we know? Fresh fruit is better for you than smoothies produced from fresh fruit. Identical chemistry, with a very different outcome. This makes liquid foods unlikely to be healthy.
Symmetry matters. A left-handed molecule can be beneficial, the right-handed version with no other change can be carcinogenic or toxic. Not necessarily, there are cases when all it does is change sweet to tart. But unless you know your stuff, you're not going to know that or how to set the correct bias.
All in all, I'd not trust biohacks of this sort for long-term use. What I would suggest, if you're interested in contributing to the science, is to get two kits, each with two swabs (mouth and gut), and a week's worth of one of these artificial diets. Take one set of samples before, then one set of samples after. Use a trustworthy DNA testing group for this. Publish the results on Steem. If the diet is as good as claimed, it will show and they deserve the PR. If it devastates your microbiome and exposes you to severe health hazards, you may have just saved the nation from a fate worse than infomercials.