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I think they are actually proper kites flying rather high (at least compared to regular sails) in approximate 300 m AGL.
The big difference is that at this hight there is significantly stronger wind.
Wanted to see what that would look like and found more info here: https://www.euronews.com/green/2021/12/20/this-huge-kite-is-dragging-cargo-ships-across-the-atlantic-ocean