It's obvious even from the message you're replying to that they meant "works" in a manner that encompasses compliance.

But don't let you already knowing that get in the way of a pointless quip implying that they need educated in thermodynamics.

If you try and fail to eat less food, the correct conclusion is not that eating less food "doesn't work"

There is more to trying than being told. Also the level of effort/sacrifice to have the same impact with vary by age and individual.

Trust me, I'm the most OCD guy ever. I literally just ate the same meal every day for 5 months. I've tried the craziest diets. Compliance is not the problem in people like me. It's something metabolic.

I tried and failed to eat less food for decades. Ultimately it was total abstinence for many hours at a time that proved to be sustainable and actually easy after 3 months.

I think it is, if it is being recommended to the person to lose weight.

You're thinking of CICO, not eating less. They have nothing to do with each other.