TL;DR: It's a logic truth, if you accept the Big Bang theory, then God couldn't exist before the beginning of the universe.

I think you didn't understand the premise here.

The premise is: "The Universe started in the Big Bang.", now, following from that premise it's impossible to have a God before the Big Bang.

You can contest the Big Bang theory and with it find a loophole for the existence of God, but the OP was finding it "amusing" and nonsensical what actually follows directly from accepting the Big Bang theory: God couldn't exist before the beginning of the Universe.