I assumed it'd be like Gmail unsend: wait a little bit before pushing the tweet through, so undo can 100% make it as if it never happened.

These are the types of features a novice app developer might implement in an afternoon after watching a tutorial video on youtube.

A company as huge as Twitter selling this as a subscription seems ridiculous to me. I don't use Twitter much, but IIRC aren't there a bunch of third party apps that have power user features like this already?

You're not paying for the feature, you're paying for the UX, which is gated behind their unusable API (so it's impossible for a 3rd party app to achieve parity with the 1st party app).

In this case it is quite easy, like delayed send third party app need not make the API call until n seconds after the tweet to give you time to undo the change.

Sure twitter may not do it client side and have special APIs etc, however ultimately they can also really do only limited time changes, other wise the tweets already read by others would start changing.

Of course they can still pull API access for violating ToS etc, however from technology context there is nothing they can do .

The point isn't that a third party UI can't do this, it's that it can't do many other things that the native UI can, so people won't even switch.