Sure, but those 1 billion people could have signed up for Twitter at any point in the last 17 years if they were interested. Twitter already maxed out its user base, so I'm skeptical that there's a 1 billion potential here.
Personally I signed up for threads and have been liking using it so far even though I never really used twitter. I think it has lower startup cost than Twitter because the algorithmic feed really works well without having to manually follow a lot of people. So I think there is reason for Zuck to dream big.
That seems kind of strange to me, though, because 1 billion is more than 3x the entire Twitter user base.