They refute themselves here. They say there isn't a massive botnet, but dismiss it by saying spam isn't unique and most of the accounts are deleted now. So, yes, it was a massive botnet. Sure, it's not uncommon and Twitter deals with them regularly, but that doesn't mean it was not hit by a massive botnet, it was.
The 35,000 accounts number was merely the total no. of accounts found from a sample of just 33 tweets. The real total number of bots would be a multiple of 35,000: <a href="http://www.businessinsider.com/twitter-fighting-a-massive-botnet-2014-3" rel="nofollow">http://www.businessinsider.com/twitter-fighting-a-massive-bo...</a>