Why does the speaker say "the new iPhone" instead of "a new iPhone"? It's because everyone expected a new iPhone to be introduced around this time. If you think your listeners know what you're talking about, you should use "the".
Use the phrase "the new ___" when talking about new products that have been eagerly expected. For example:
Have you heard the new Katy Perry album?
I can't wait for the new Harry Potter movie.