The compiler output is a tree of runtime.Module. DSO-exportable means a runtime.Module in that tree whose type_key is c or llvm. TVM links directly against LLVM and invokes the LLVM APIs to generate code.
When you call export_library, TVM traverses the tree of runtime.Module. When it encounters one with type_key of llvm, it uses LLVM APIs to produce binary .o for the targeted platform, then passes all such encountered modules to the system linker to produce a .so.