This is an inaccurate take. It loiters on the bottom in an energy-efficient sleep mode.
Separately, it collects small amounts of energy via geothermal means similar to something like regenerative braking. This, of course, allows it to sleep for longer periods.
From both a defense publication article, written by the Program Manager, Kyle Woerner, and reading up on the company purportedly behind the geothermal research. They go into a bit more detail.
If I can find online links, I’ll post a follow up comment with them.
Separately, it collects small amounts of energy via geothermal means similar to something like regenerative braking. This, of course, allows it to sleep for longer periods.