Only 13% of emails are written by people

In this case they're saying "created by", as opposed to "a copy was received". The latter would be weighted much more heavily towards automation.

In other words, I think a human-written message to 10,000 customers would be counted here as +1 to "written by people", whereas a "Your account X is overdue" generated for one customer would be +1 to the not-by-people.