I guess it's good to be cynical at times, but do you really think the tweeter and indeed Facebook would put their reputations on the line to gather such a small, insignificant information?
It may seem small and insignificant to us, but how much do you think advertisers would pay to see the link that I just pasted into an iMessage, or the contents of the email I just copied?
They would pay significantly less than the billions of dollars Facebook stands to lose if they were found out to do this.
How could this cost them billions? They do lots of things that are pretty gross. Does this cross some sort of line they haven't crossed before?
I doubt he's lying, but you have to trust that what he's saying is accurate and stays accurate. Some helpful dev that doesn't get the privacy issues could switch it to process the clipboard on the server, or accidentally add some logging code which happens to log that value remotely.
I'm more concerned that iOS gives this ability without any warning or notification to the user, because even if Facebook is virtuous (now) about this, other future apps may be less so.