I have to agree. I'm not saying that people on liquid diets don't have digestion problems, but the liquid diet alone likely didn't cause it.
Even if you eat no food, you'll still have bowel movements as your intestines shed cells and mucus. If you have a colostomy done (intestines no longer connect to your rectum) you'll still pass small small amounts of stool out the normal way (anus).
Your bowels are muscle. Like any muscle, if it doesn't get used much, it wastes. Just because you're passing some stool, doesn't mean you're in good digestive shape, no more than an irregular heartbeat is healthy "because at least the heart is beating"
What does "good digestive shape" even mean? I've never heard of "digestive muscle wasting" in any of the medical literature. Your digestive muscles are smooth muscle (different from the heart) that are autonomically innervated (more than just food stimulates them).
The fact that people have gone on extended fasts and then broke them without much trouble suggests a "breakdown" of digestive function is a weak argument.
I would say that a change in the biome of the digestive tract is probably more responsible for any digestive upset after breaking a fast than "muscle breakdown".
Even if you eat no food, you'll still have bowel movements as your intestines shed cells and mucus. If you have a colostomy done (intestines no longer connect to your rectum) you'll still pass small small amounts of stool out the normal way (anus).