Yes and no, just like any major media platform, huge majority of tweets being seen are from a very small group of influencers/popular person. That's why when you join twitter, it suggests to you a lot of people to follow that are already big.

There's only a yes in your answer.

No. You can have only 10% of accounts actively tweeting and the rest just consuming what those post. All those - active and not - are monetizable

You don't really need that many people to submit content though. I imagine most YouTube users have never uploaded a single video, and they don't need to, since there's basically no end to available content there.

Twitter specifically added the annoying feature of your likes being shown to your followers so that lurkers would be actively contributing to the algorithm though.

As long as lurkers are "liking" content, their local network will see an engagement increase.

This is a second-order objective though. The goal is to show ads to humans on the platform. Having a lot of human authors (or any kind of content authors) generating content is a way to achieve the goal, not a goal in itself.

There are other ways to achieve the goal, such as making ads more relevant (targeted advertising), having users consume more of the same content (recommendation), having the same content take longer to consume (periscope). Growing the number of human posters is definitely not a requirement.

The people who create content do it in such massive amounts that this never seems to be an issue.