Email contents and email addresses are both text. Yes, the concept still applies. The point is that email contents are almost never the same to begin with. If we're including the timestamp, then every email is almost unique by default. Mentioning that the email addresses are unique is making the point that we've identified just as many people [1], which is interesting. The statement "these 3.3 billion emails are unique" is much less interesting, because we've identified messages and not people. Also, people are usually more concerned with the information in the email rather than the count. Most/a lot of the value of an email comes from the information in it.
If I were to release 3.3 billion emails between random low-profile office workers (let's say) which contain nothing interesting, I'm not so sure that would make a headline.
If I were to release 3.3 billion emails between random low-profile office workers (let's say) which contain nothing interesting, I'm not so sure that would make a headline.
[1] just as many or THEREABOUT*