Yes, you are correct that sediment export results are provided as tons/pixel/year. As stated in the Introduction of the SDR chapter of the User Guide the “Outputs from the sediment model include the sediment load delivered to the stream at an ANNUAL time scale, as well as the amount of sediment eroded in the catchment and retained by vegetation and topographic features.” And the Sediment Delivery section states, “For each pixel, the model first computes the amount of ANNUAL soil loss from that pixel, then computes the sediment delivery ratio (SDR)…”.
See also, the Annual Soil Loss section and “Time considerations” under Quantitative Valuation which clarifies that, “…the SDR model represents only average annual impacts under steady state conditions.”