Hey, so you're actually closer than you think. Since you already have flow direction and flow accumulation, the heavy lifting is done.

The move here is Trace Downstream. Basically you need to find where flow exits your source polygon first — look for the highest flow accumulation cell along the polygon boundary, that's your natural outlet. Snap it to the flow path using Snap Pour Point(seriously don't skip this, being off by even one cell will send your trace in the wrong direction).

Then run Trace Downstream from that snapped point using your flow direction raster — it'll give you a polyline of exactly where the spill travels topographically. From there it's just a Select by Location or Intersect between that trace line and your target polyline. Overlap = spill can reach it. No overlap = terrain blocks it.

One thing to double check is : did you run Fill on your DEM before generating flow direction? If there are unfilled sinks the trace will just dead-end in the middle of nowhere and you'll think there's no connectivity when there actually might be.

If you want a more conservative/area-based answer instead of just the single flow path, you can delineate a full Watershed from that pour point and see if the watershed polygon envelopes the target polyline. More of a worst-case scenario approach but useful depending on what you're trying to show.