I had the same initial thought as well, but I realized this caveat would only be true if dams just made rivers deeper. But dams flood valleys and they make the water body much wider, not just deeper. A river that was 300 feet wide becomes a lake that’s miles wide. The width increase (10-100x) completely overwhelms the depth increase (maybe 3-10x), so surface area increases substantially.

Given that Earth averages towards smooth, the rate of width increase overwhelms the rate of depth increase when dams fill.

The only exceptions might be narrow canyon dams with nowhere to spread laterally, or dams on already-wide rivers. But those are rare.