At the time this was written, that lower threshold was the way people were describing the reason for the Twitter constraint. Since that time, the history of Twitter has been clarified a bit more:

Back then, we had no character limit on our system. Messages longer than 160 characters (the common SMS carrier limit) were split into multiple texts and delivered (somewhat) sequentially. There were other bugs, and a mounting SMS bill. The team decided to place a limit on the number of characters that would go out via SMS for each post. They settled on 140, in order to leave room for the username and the colon in front of the message. In February of 2007 @Jack wrote something which inspired me to get started on this project: “One could change the world with one hundred and forty characters.”
(from @dom’s great account of the early history)