The claim is usually that A) people have an inflexible appetite and will be increasingly driven to eat while at a caloric deficit, B) at some point this is almost involuntary and that point differs between people (e.g. consider not eating for three weeks; some people can do it on a hunger strike or a fast, but it really sucks), and C) some people have a slower metabolism which puts them under greater pressure to eat.
I don't disagree that calorie restriction is effective. I just think that you're dropping appetite from the equation when GLP-1 agonists have proven that reducing appetite is effective too.
I'd caution: GLPs reduce your appetite, but they do not fix your diet.
I am not anti-GLP, someone who believes using GLP drugs is cheating and people should achieve that with blood, tears, and sweat. I see it as a "tool" in the arsenal. I've no use for GLP as I've no problem sustaining deficits as large as 600-700kcal for long. But I've met people who can't do this, so for them GLP is a valid choice, but I'd recommend they pair it with proper dieting and lifestyle changes.
You want to eat good stuff and avoid stuff not good for you; blanket reducing appetite may result in loss of both good and bad.
Here's what GLP users say about the macrocodex method: https://www.reddit.com/r/tirzepatidecompound/comments/1omfgx...
The guide linked in the above post is macrocodex's prototype method, which was later formalized into an app.
I don't want to make strong claims on this, but using GLPs may increase the risk of developing gallbladder stones, as GLPs make it very easy to eat far less than necessary.
A proper macro and calorie balanced diet can give you better results with a smaller GLP dose.
Use GLPs if your actual problem is staying on a "sustainable deficit." Many people who use macrocodex report they do not feel hungry when on such a small deficit, but there are a few who are hungry even at a deficit as low as 300 kcal; for them, GLPs are more suitable, but not a replacement for proper dieting and lifestyle changes.
Most importantly for A) is dropping sugar. Calorie restriction doesn't have to mean being hungry all the time. First drop sugar, you'll be hungry all the time until the withdrawal subsides, then eat at a slight calorie deficit. It doesn't feel different than a slight calorie surplus at this point. I do that and I am in complete control over my weight. This whole discussion seems totally misguided to me.