It's not very useless. The title suggests a perpetual motion machine, so I was wondering what that was doing on HN. Knowing you have to lift it first (from the article or from the comments) is important to know and lets one think this isn't necessarily made up.

I don't think that's useless pedanticism at all. If you didn't know anything about physics, you're likely to hear "powered by gravity" and think "great, gravity is ubiquitous and free, so this is totally free energy", whereas actually these poor people will have to eat more in order to light their homes. So it's important to communicate what you actually have to do to power this light.

On the other hand, your "if you're reductive at all, you have to be 100% reductive all the time or shut up" argument is completely unhelpful.