Twitter do not allow editing in their API, so all this app is doing is _not posting it_ for a short period of time after you click submit to allow you to edit if you need to. No one will see the tweet until that time period is over.
"Undo or redo a tweet Discard your last tweet or re-edit it, quickly with shortcuts" or are you just guessing it based on the twitter API?
As another example, Gmail has an "undo" button on sending an email, but other email clients don't (mostly). Why? Because email doesn't have the concept of un-doing a send, so the only way they can implement this at the "protocol level" is to... not, and to implement it entirely in the client with a delay.