Mesh edge won't join from extracted curves

So I have a mesh which I got from a displacement map on a plane, and now I’m trying to make it a solid mesh.

I used grasshopper to extract the open edges of the mesh

and then lofted the curve together to try a make a mesh that I can join and then just put a cap on.
But when I try to join the lofted mesh with the original, it keeps saying that there are open edges where there clearly aren’t. What am I doing wrong?

see this video-