Happy to see that you found the new feature Though the disclaimer comes first: You are on experimental terrain!
Now the solution to your issue. Unfortunately, PyROOT (so the Python bindings for C++) are not (yet) able to parse the template we use in C++ (see here). However, you can play a little trick to make it still working in Python! Because PyROOT sees all objects created by the C++ interpreter cling, you can just make a call in C++. Please note that the snipplet assumes that you have run the tutorial
computeModel = TMVA::Experimental::Compute<4, float>(model);
df = ROOT.RDataFrame('TreeS', 'http://root.cern.ch/files/tmva_class_example.root')
df = df.Define('y', ROOT.computeModel, ROOT.model.GetVariableNames())
h = df.Histo1D('y')