I found articles about this experiment, but it doesn't appear that an actual study was performed or formal paper published about it. Although that person was (perhaps still is) a professor in nutrition, this Twinkie story remains an anecdote. Anecdotes are worth something, but what that means is we're missing a bunch of variables, especially what he was eating prior to the experiment and whether eating nothing but Twinkies and other snack foods also lead to muscle loss. It's also totally unclear, from what I'm reading, how much exercise he was performing and what is TDEE was prior to and after the weight loss. A month is not a lot of time to understand the long term sustainability of a diet like that.
If a person finds that a Twinkie diet works to help them cut fat in a short period of time, there's really not much to object to. Asserting that it's OK to just eat food that is classically categorized as "junk" as long as you don't eat too many calories is fallacious and ignores the poisonous nature of the things we eat.
> But you're right, a calorie definitely is a calorie - even in a diet.
It's not that calories aren't what they are, or that they don't have some use. The core objection is that they aren't an appropriate measure for human health. At best, they are a very imperfect proxy for mass measurement. If you want to lose weight, don't eat more anhydrous mass than you eliminate. When you measure calories to the exclusion of nutrient composition, not only is one ignoring a mode of optimization (that could lead to longer term successful outcomes), but they are relying on very inaccurate tools. Calorie counts on packaging are allowed to be as inaccurate as 20%, and the tools people use to count their "calories" really aren't accurate enough for that purpose (researchers still use respiratory gas analysis for that reason).
Obviously, someone can maintain or lose weight by counting "calories." That does not mean what they are doing is optimal or sustainable in the long term, or that anyone is a fool for taking the exact opposite approach. The many people out there who do not count calories, eat as much as they want of a specific diet, incidentally eat more in "calories", and still lose weight throw a wrench into the whole "it's just calories in, calories out" hypothesis.