An extrude by a single tangent direction won't be quite right because the tangents are slightly different on each side. That's because the surface is cut at an angle as described above. They're only a little bit different though since the surface is only very slightly bent so it will be hard to tell visually.

But if you want a precise result then doing the untrim will recover a planar ending edge that can be extruded to get an accurate tangent extension.

- Michael