Your question is incomplete
I think you're putting too much stock in the novel you read — or perhaps you're not telling us enough about the context of the statement. At a guess, the author was probably trying to use a cool conjecture of physionomy to foil a crisis in the novel. Writing fiction is much, much more than just being "scientifically real" (which I don't put a lot of stock in, anyway...). Why am I making a big deal of this?
Because the point that author was making only has meaning if eating the food was for sustenance.
I've an older sister who once was visiting the home of a family she didn't know well. She was invited to drink something hot and was asked if she wanted some milk to cool it. When my sister said yes, the wife took up her baby's bottle and squirted milk into my sister's cup. "And, oh Mom... I drank it!" declared my sister in a letter home.
In other words, WHY someone is eating something matters to the context of your question. Humans can eat all kinds of things without dying or even getting particularly hurt — and yet derive not a single mole of nutrition from the eating. Why would we do something like that? Perhaps as a courtesy to the alien family we're visiting... a family that doesn't really understand our dietary needs and is only trying to be polite.
What works in your favor is that chemistry isn't alien
Sometimes we think DNA and such are mysterious and magical. It's unlikely, we think, that the DNA we find on Earth would be found on a planet with an independent evolution. Consequently we believe that we couldn't eat the plants grown on that planet. Indeed, we suspect they'd be poisonous.
Maybe...
It's certain that we know that some types of chemistry are beneficial to the body, some are detrimental, some require massive amounts to be one or the other, and yet others have no effect at all. And that's just here on Earth.
We also know that the chemistry that makes up Earth is the same chemistry that's all over in the Universe. Does that mean there couldn't be an ammonia-based lifeform? Or silicon-based? Or something else? No... only that their chemistry must conform to the properties of the Periodic Table of the Elements just as everything must do here on Earth.
Which means that your quest for scientific realism is, if you'll excuse my frankness, misguided. Your iron-based-blood creature will have trouble eating meat from a copper-based-blood creature if you want it to be so. Nothing more. Nothing less. Because that's the way it is in real life.
Still not convinced?
There are humans who are lactose-intolerant... and those who can freely benefit from consuming it. There are humans who are allergic to peanuts... and those who can freely benefit from consuming them. There are humans who are allergic to water, and yet they need water just to live. The physiology of nutrition and digestion is not in any way binary. Some people need a lot of protein to be healthy, others need very little. Some people can't gain weight to save their souls in heaven. Others gain ten pounds looking at a picture of chocolate cheesecake. You can make yourself tolerant to some poisons by training your body to them. Others can't eat some foods because of what those foods look like, taste like, or the texture of the food — the issue having nothing to do with nutrition.
So what's reality, really?
Far too many people forget that the fundamental truth of science is the necessity — the necessity — that facts must be proven through repeatable empirical experimentation. Mathematical prediction is not science. A clever YouTube video or fun insight in a novel is not science. And what's real is much more forgiving than the current fad of scientific realism permits.
Unless you plan to change chemistry in your universe, the answer to your question is, "it's believable that one can eat the other." Notably because anyone who disagrees can't show a repeatable experiment to prove you wrong. Of course, they couldn't do that if you'd chosen another of your three bullets, either. But that's both beside the point... and the point.
Trying to build a fantastic world pursuant to what we know about science today (because it won't be entirely true according to what we know about science tomorrow) is noble — but good worldbuilding is more concerned with consistency and suspension of disbelief, and if you can't suspend it using the Scientific Method, then suspend it with conviction that your idea it great.