I think the problem is that "calories in, calories out" is not that easy since it's hard to measure the calories in and the calories out precisely. "Calories out" especially, as most people will just tell people to use a TDEE calculator or some other approximation. When you're then asking them to have a 500 calories deficit, it is very possible that it won't work for them as they (as in, the tool they used) overestimated the "calories out" part.

Edit: to expand on that, "calories in, calories out" is a classic motte and bailey (https://en.wikipedia.org/wiki/Motte-and-bailey_fallacy). Here the motte is "it's easy for anyone to lose weight, just input a few numbers in a few apps". The bailey is "calories in, calories out is scientifically true". I've never seen advocates of calories in, calories out offering a more precise way to measure the consumption of calories than a TDEE calculator that you can find online, where you usually input your weight, height, age and sex. The "calories in" part is easier to deal with, as if you measure everything you put in your mouth, you have the upper bound of the calories you can absorb. Still, the calories out are hard to measure reliably, especially in people that already have a slower metabolism than others (which makes it easier for them to gain weight in the first place).

There's also the distinction that "calorie intake" is often meant as "what you put into your mouth" but is actually "what you actually absorb".