I find this painful to read. It's always about what Stephen Wolfram has done and how it is realised in Mathematica. If you are a true expert in a field you should have less trouble acknowledging other people's contributions. I missed, for example, the connection with Church numerals or the acknowledgement that substitution is not just defined in Mathematica.

If you want to learn about combinators and have some great fun while doing it I just can't recommend enough of this book: "To Mock a Mockingbird" by Raymond Smullyan. Don't sweat it, buy it, I haven't had so much fun in a long time.