Wouldn't it just burn more calories?

Isn't it just exercise? Unclear what the concern would be despite in general agreeing with you on human body being hard to change without side effects

Did you read even the first paragraph of the link? This is about using skin as a medium of power distribution to devices, not harvesting power from the human body. No hand-wringing required.

Makes sense...basically the energy equivalent of Herbert's https://dune.fandom.com/wiki/Stillsuit

It's obviously useful right? Currently I have to remove my smartwatch to charge it for a tiny example

The paper answers this. It's about powering a suite of sensors and devices that can also communicate using your skin as the distribution system for power and comms. I could see the appeal in a world where many such sensors and devices might exist on a single body.