They didn't mean it was literally a slow metabolism. They meant that what the article is about is often refered to as a "slow metabolism". It's a misnomer since that is not the mechanism but there is definitely a phenomenon at play, which is what the article is about, the actual phenomenon rather than the bro science.

There is also a conflation of a slowing metabolism and low energy availability, which can reduce the amount of energy expended during the day (because you feel tired and do less). It can be quite subtle but when I've done some extended periods in a calorie deficit I start to notice subtle things, like a propensity to sit a bit longer, or to reduce my overall body movements. My resting metabolism is the same (I've had it measured a few times) but my body looks for ways to expend less energy.

Anecdotally (but an experience shared at by at least some other long distance runners), when I get quite far into a calorie deficit via exercise, my brain will start suggesting shortcuts - urges to cut the corner on a trail, take a shorter path back etc. Its quite interesting!

Considering the amount of effort it traditionally took to get food it's a shame that our brains would want us to be less active when it could otherwise start suggesting that we be even more active to find/hunt food.

The suggestions generally still work towards my goal; this is a different phenomena to "holy shit the consciousness is trying to kill us again" panic switch. Maybe its some low level optimisation to reduce energy expenditure, so we can last longer, and find even more food!

More than that, your body downregulates unconscious activity (known as NEAT). This reduces fidgeting and even influences how likely you are to get up, say if you're thirsty or have to pee. In a caloric deficit, you're more likely to just "decide" to not get up to get that drink.

I think you missed the point that he said "slow metabolism" is just a colloquial term for something else.

them and the rest of society