Its not even that new, REDstack started a engineering scale trial of salt-potential electricity generation already a few years ago. (https://www.theafsluitdijk.com/projecten/blue-energy/wat-is-...)

Another non-obvious (to me) thing it needs is the ability to drain the brackish water you’re creating. At scale this probably involves at least a long canal or pipe, and makes the setup more difficult that “let’s just put this were the river meets the sea”