Just because one rower on a relief has his knees raised a little? Colour me skeptical.
In any case, the sliding seat does not necessarily improve performance, only when using modern, light-weight boats and only on 'flat'water (lakes, canals).
That's why you generally won't see them in open water rowing craft. As you said, it was an invention born out of competitive rowing, sliding seats require much more energy to operate.
It really doesn't make a lot of sense for long distance, seagoing vessels.