Japan has opened its first osmotic power plant

This energy is not free. Solar cells and wind embody the cost of production of the device as the input cost along with cost of construction, and transmission, but the primary energy input is predicated on a real externality: Wind and Sun.

This system depends on using a LOT of energy to maintain an osmotic pressure gradient. That it turn depends on pumping water across a boundary. Energy has to be expended. Now, if you run a de-salination plant and/or waste water treatment you have to expend MOST of this cost anyway, so you are scavenging energy back from an unavoidable, non-externality cost.

This is a big difference. Wind and Solar bring energy in from the Sun and weather, outside human expenditure. This brings BACK some expended energy, doing another job.

I suppose hypothetically, given immensely saline water CLOSE to less saline water you could expend significantly less energy to arrive at the boundary condition but its for kilowatts, not gigawatts or even megawatts. The places which have these conditions might also have high sunlight or wind conditions no?

I think it's meant to help reduce the salinity of waste from the desalination plant in a process that recovers some energy to make the whole system a bit more efficient.