I'm not going to repeat what others have said about 600 tweets being a really small number, but it seems that this "rate limite" thing is actually a cover up.
Basically he's been trying to make it mandatory to have an account to view twitter and that function was put up just hours before the tweet about rate limiting...
Thing is, the blocking non logged in users from seeing tweets thing is horribly coded. Every time a user tries to look at any tweet from a browser it bugs out and sends ten of page requests per second to try and display the page but obviously the "if user not logged in deny service" stops it. In other words: twitter is DDOSing itself constantly on a massive scale, DDOS being the most basic form hackers use to crash websites.
I predict that he's going to keep going with the "rate limite" until the signed out user bug is fixed, and then he'll Come back with something like "After listening to user feedback we've rolled back the feature"... The fact that it says "temporairy" also leads me to believe that.