I wouldn't say it's clear by any means but follow the "in 2017" link. After reading that page if it doesn't make sense why simply leaving those fields out wouldn't work follow a few more links in any of the articles and then try rereading the "in 2017" link.
It's not so much "is it possible to push arbitrary text in RSS" as it is "is it possible to push the writing flow in RSS".
I've been writing about this for a while, I don't repeat everything in every post, which is probably why it was confusing.
Anyway, what Twitter did is repeat the body of the tweet in the title and description sub-elements of item.
The reason they did this is probably that the dominant reader of the day, Google Reader, pretty much required titles. So when you'd read a tweet in a feed reader you'd see the text of the tweet twice. Not a good user experience.
The problem isn't with RSS, because it allows for titleless items, rather with the reader.
And that problem is still with us today because there isn't much consistency among the readers other than the Google Reader model. They are all following GR, not RSS.
And that makes tweet-like-things-in-RSS pretty much a non-starter.
For examples, look at my blog on any given day most of what's there is too short to have a title, like a tweet.
It's not so much "is it possible to push arbitrary text in RSS" as it is "is it possible to push the writing flow in RSS".