That seems odd to me - sure in the water rowing is slower (friction is way higher), but in terms of muscle use and energy output, rowing uses way more muscles and gets better range of motion in the legs.
I'm not much of a rower, but after your pull, pulling your body forward for the next row is all wasted effort as far as producing mechanical work. Cycling doesn't really have any wasted phase like this.
Here's a bunch of rowers comparing their cycling/rowing watts: https://www.c2forum.com/viewtopic.php?f=31&t=186612
What I'm not sure is if you were to measure total energy expenditure at the human (as opposed to the spinning wheel), then cycling/rowing might be closer.