SDL is a layer above OpenGL; in fact it uses GDI on Windows by default and also has a DirectX backend. People are probably saying you can use OpenGL to get around limitations of SDL on platforms that by default use OpenGL (ahem, Linux) because the higher level abstraction doesn’t expose that functionality. However, then your code is “less” portable to Windows (or at least Windows using the GDI backend).
Also, SDL does a lot of other things besides graphics – audio, input, etc. that OpenGL doesn’t do.