If you want someone to gain weight, do the same calc, tell 'em to eat 100 more calories. Not gaining? 200. Really not gaining? Wow, fast metabolism, shock 'em with 500 extra. Anecdata, but the only time I saw someone not slowly gain on 200 extra, there ended up being a lot of walking we weren't accounting for in our maintenance cal calculation. Our O in CICO was off. For reference, 200 cals is like a 16 oz of coke, or 2-3 apples. Decently precise.
The variance between a fast and slow metabolizer isn't 20% of daily calorie burn, maybe 10% on the high end. Guesstimation, probably more like 5%. Well within the tolerances that are used for weight modulation (up or down).
If it weren't the case, why are so many body builders (natty or enhanced) able to run repeated bulking and cutting cycles? Are they genetic outliers?
Can you calculate it 100%? Nope -- so many variables, probably tons we don't even know about yet. But it's accurate enough to be very effective, and very simple to implement.
I'm sort of with you, but it sounds more to me like: start eating less, and you'll lose weight; eat more, and you'll gain weight. There's just too many variables here that change how food is metabolized and digested; including the microbiome which CICO ignores.
the other answers here are more detailed, I just wanted to state it simply.
Again, the digestion point seems to reinforce CICO. If you gain weight because your digestion is slower (let’s grant the point), it’s not because your body some how treats calories differently, it’s that you’re getting more calories from your food. Your calories in are higher.
Not to mention that diet products have existed to game your digestive system to absorb less calories from your food. With the rationale being… less CI in CICO. See also: folks abusing laxatives for this purpose
