From http://docs.scipy.org/doc/numpy/user/building.html

> Various NumPy modules use FORTRAN 77 libraries

    $ pip show tensorflow theano
    
    Name: tensorflow
    Version: 0.7.1
    ...
    Requires: six, protobuf, wheel, numpy
    ---
    Name: Theano
    Version: 0.8.2
    ...
    Requires: six, scipy, numpy

You are right that arithmetic in both of them is not written on Fortran, but the packages do use NumPy as you can see in the pip output.