Key word is “import them directly from the editor”, no Blender used. When using the editor, the import process calls AssetImporter. Mixamo now offers two fbx formats to download, 1) an fbx format and 2) FBX for Unity(.fbx). The first is probably a FBX 2016 format and the latter is most likely FBX 2013. Assimp still only supports FBX 2013, so the logical choice is to choose FBX for Unity.
Just looked at both FBX formats in binary:
- FBX file = FBX SDK/FBX Plugins version 2014.2.1
- FBX for Unity = FBX SDK/FBX Plugins version 2014.2.1 build=20131219
So, I still don’t address why it doesn’t work with Blender (someone else might have that answer as I don’t use Blender), but my point is you can use the Urho3D editor to import the fbx files directly and they’ll work, provided you download the FBX for Unity files.