why would there already be footage for the batman solo movie?
Look at the suit, there are many little things, like the bandana twirling that could be cgi. This could be a lighting test to see how it all looks in an environment, which is why it has different light sources behind him. You do this, so when the actual filming starts, your assets already ready, and you can kind of plunk it in and then work with it instead of having to worry about lighting and whatnot in post.