Here's what the tweet in question said:
19 million my ass #biebermyballs
It was potentially seen by Bieber's 19 million followers, or even more, given that Twitter is a mainly public social network. The remark was aimed at the fact that Bieber today crossed the 19 million follower milestone. The tweet was subsequently deleted. Either the hacker in question deleted it right after posting it, or (more likely) someone else was active on the account at the time, saw it, and deleted it.
Furthermore, the hacker also started unfollowing some of the Twitter users who Bieber follows (we're talking over 100,000 people here) and then blocking them. Bieber's team reportedly worked quickly to follow everyone back once they regained complete control of the account. Bieber's account may be verified, but if someone gains access to a verified Twitter account, it doesn't become unverified.
If the hashtag "biebermyballs" sounds familiar, that's either because you're a Bieber hater or you saw it tweeted repeatedly from Carly Rae Jepsen's Twitter account when it was hacked several weeks ago. It's not clear if Bieber's account was hacked by the same perpetrators or if this is merely a coincidence. Jepsen's hacker was never identified, and I wouldn't be surprised if Bieber's gets away with it as well.
If you have time to kill, read the tweets for the Twitter search "19 million my ass #biebermyballs." Some of them are hilarious.