Unfortunately the article has no information about actual power generation apart from:
Their devices can generate about 1 volt of energy for every square centimeter of skin space
Which doesn’t make any sense.
The paper it self says
A record-high open-circuit voltage among flexible TEGs is achieved, reaching 1 V/cm2 at a temperature difference of 95 K.
Which seems to be a very high temperature gradient, not what you see in daily use.
For the surface area of a typical sports wristband (6 cm by 25 cm), a power output of 12.5 μW and a voltage output of 5 V can be generated when the wearer is walking, which is enough to directly drive most low-power sensor nodes with radio frequency communication.
This is the most relevant information, 12.5uW is pretty good and could run some very low power devices. Hopefully this technology reaches mainstream one day so we can get rid of the need for batteries and chargers.