Simple is not the same thing as easy. Mostly "diet plans" are basically psychological tools to help you do a difficult but simple thing.
For me, it is that simple. My point was that for my wife, there are a ton of other considerations she deals with. Sure, they're mainly in her head (though possibly other factors such as gut biome play a role), but large she has more details to factor in when she thinks about "will I be able to do this?". I on the other hand don't. For me, it's exactly as the author described, simple.
Yes, simple does not mean easy, but saying quitting smoking is simple to a smoker is not an honest assessment of their situation.
Most things in life are simple if you ignore reality and speak of life from a bubble.
Somebodies reaction to a hard thing may be complex, but that is more about them than the thing. Although as you note, there is a risk of oversimplifying (e.g. your example of possible implications of gut biome, but that doesn't really shift the fundamentals).
The reason I think this distinction is a good one to make is the problem with the converse. If something being hard implies it is complex, it will lead often to people generating extraneous complexity around it which makes it actually more difficult for the people involved, and can lead to very convoluted thinking and errors in that thinking. You need only look at the massive industries built around things like weight loss and quitting smoking to see examples of this (not that everything in those industries is nonsense, but they do contain a lot of profitable nonsense).
This had nothing to do with bubbles. The risk is that instead of supporting people in doing a hard thing, we raise false hope of making it easy.
That's why I felt my situation and my wife's differed. Mine was simple because there wasn't an underlying pile of complexity. Her underlying "addiction" and etc are noteworthy items in her list of things she needs to do. It's simple if you ignore the problems, but if she wants to actually implement that simple instruction she needs to figure out the little details that affect her.
So yes, it is simple if we're not talking about her situation with context and reality. Mine is simple in all contexts, in my opinion.
Smoking is a good example imo on where our opinions differ. Smoking is super far simple in my mind, because many people can't just stop smoking (or rather, lack the willpower). So they need additional data, the simple instruction was.. imo, not so simple.
You are basically promoting the idea that I am rejecting, that because people find things difficult and have a lot of complicated reactions to it, that means the underlying problem is itself must be complex. I don't believe that is either true or useful, and argue that it can often make the problems themselves worse by muddying the water.
Some problems are complicated but relatively easy. In this case, if you just find "the right trick" you can solve it without much difficultly. We do people a disservice when we pretend that simple-but-hard problems are actually complicated-but-easy. This is something that is often done, for commercial gain, in industries like weight loss and smoking cessation. At core, I am arguing that this is a bad thing.
At any rate, we've probably reached diminishing returns here.