They know when it's displayed to you. Let's say that Bob follows Sally (and a bunch of other people)
* Sally makes a tweet at 8AM
* Bob doesn't check Twitter until noon
* Bob doesn't scroll down, the tweet at the bottom of the screen is timestamped 11:30AM
In this scenario Twitter never displays Sally's tweet in Bob's timeline. It is thus not an impression. If Bob had scrolled down to see tweets going back to 8AM Twitter would display Sally's tweet in Bob's timeline, this would count as an impression.
If you're asking if they know that you actually read, parsed, comprehended, or contemplated the text of the tweet then no of course not. Likewise somebody clicking on a link doesn't mean they actually read, parsed, comprehended, or contemplated that content.
Of course, and that's how ad impressions are counted. But that doesn't help me as a user, and advertisers pay much less for "impressions" than for an ad that is interacted with in some way. So that's worse for everyone than an ad that you click to dismiss.
