Of course he rehearsed every keynote. You would be a fool to do any performance without some kind of rehearsal, unless it’s improv.

You can search YouTube for Steve Jobs demo fail for plenty of examples of it not going smoothly. He practiced but he still did live demos and had plenty of opportunities to mess up

If this was a presentation, sure, but here he’s answering questions in a room full of journalists, surrounded by other Apple employees with specialised knowledge. It seems likely he really was pondering the right words to answer.