This question already has an answer here:
What's the purpose of tf.app.flags in TensorFlow?
This is google's way for parsing arguments from the commandline. Have a look at python-gflags. As far as I'm aware, google is the primary user of this commandline parsing library. The rest of the world uses
argparse these days.
But basically, the "tl;dr;" is that you're right -- They are setting up global data. However, it's global data that can be fiddled with via the commandline.