Take an air squat (i.e., a squat with only your bodyweight for resistance), for example. If you do 100-200 reps of those, there’s a solid chance you’re going to have some significant soreness the next day or so, and this is because the eccentric phase (the descent of the squat) is loaded, i.e., gravity pulls on your body and thus pulls you downward. You have to resist the load of your own bodyweight - otherwise, you’d collapse down onto the floor.
It turns out that it’s this loaded aspect of the eccentric phase that causes the soreness.
In rowing, the equivalent of the squat’s descent is called the recovery, and it’s when you pull yourself back to the starting position, or the “catch” position. Here’s the key part - unlike the squat, wherein gravity pulls you down, in rowing, you have to pull yourself back to this position. Thus the eccentric phase is not loaded, which means 20 minutes of hard rowing doesn’t wreck you the same way that 20 minutes of air squats does.
As a side note, both biking and pushing a prowler or sled are good options as well as they, too, lack a loaded eccentric phase.