I think your intuition is right, and I'd agree that the extensive regions where what looks like quite elevated terrain ends abruptly at the coast--both the inland sea and also the southern coast--seems somewhat unlikely. I'm not sure what your elevation scale is, but if these are multi-100 m cliffs that otherwise appear well-eroded, I guess I'd expect more in the way of coastal plains / lowlands, even if just a narrow strip.

On a more practical note, I'm actually curious how you managed to get such cliffs? Except where I just manually masked out the seas, in my hands Wilbur always wanted to give me coastal plains, even when I didn't want them!