Indeed Rhino3dmIO is open-source, but that supports only reading and writing of the open 3dm file format and “simple” geometry operations that are part of the OpenNURBS library. The more advanced geometry operations, like
Mesh.CreateFromBrep are only supported inside Rhino, because this uses the closed-source part. These more advanced operations also include brep and mesh booleans, curve intersections, closest-point finding, etc. etc.
So yes, you can use the 3dm file format and the whole geometry data structure + simple operations that comes with Rhino3dmIO, but you can’t use the more advanced functions. Think of the McNeel perspective: if they would open-source Rhino completely, another company could use their algorithms for these advanced functions and Rhino sales would most probably be affected. These algorithms are typically developed and fine-tuned over years.