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Yes, that's more or less how it was done. The explanation is here: http://www.spinellis.gr/blog/20090805/index.html

I'd say that's all it took, a lot of automated tweeting with better guesses.

I would imagine posting a bunch of tweets at once with the same number, and deleting the ones which didn't match. That's assuming that deleted tweets completely disappear though.

It involves multiple redirects. One goes to http://simonsarris.com/recurse, which surely can be modified at any time. So, goo.gl short link to a controlled resource that shortly afterward is made to point back to the tweet.

The post I was replying to wasn't the goo.gl one; it was the @selfrefer tweet link, which has a link to the tweet at twitter.com in a tweet: http://twitter.com/SelfRefer/status/3128391843

Can you edit goo.gl URLs?