Note that they used a pretty old+forked version of keras for the mxnet backend
Sorry that's super confusing, since currently Tensorflow is 1 of 2 possible backends to Keras. Is this the same Keras?
One of four (MXNet and CNTK alongside TF and Theano), and the Amazon deep learning API forked Keras to default to MXNet support before it was really ready - which irked the Keras authors quite a bit.