The very next sentence is “But in shallow waters, there's a rapid route to the atmosphere and methane escapes before it is oxidized.”
Most sediment collected by dams ends up there because it weighs more than water and so filters out as soon as the water stops flowing. As rivers reach the ocean that process generally produces vast and very shallow silt deposits. https://en.wikipedia.org/wiki/Estuary You can even read about what happens in dam removal when that same sediment flows down stream and ends up very near the surface. There are some very striking images of this processes when muddy flood water reaches the ocean.
PS: Also by deep water their talking miles deep. Dead fish end up falling miles in most of the ocean.
> In deep waters, methane is likely to be oxidized as it travels its long route from the seafloor to the atmosphere.
Coastal waters emissions does not imply all biomass. Open system — what escapes to deep waters stays there. Quote above implies dam water is warmer.