The WTS is able to make such bald assertions because people in general remain ignorant of the reality of the complex situation during the Church's early centuries. I do not know where they got those figures but their simplistic description belies the realities. As an example, during that period professed Christians were killed by other Christians, including beheadings, because of their doctrinal differences. Not unlike the current internal ructions within the Islamic sects.
In some instances, Christians begged the courts to sentence them to death, as they wished to share in Jesus' death.
Good books on the complex politics at play include: "Jesus Wars: How Four Patriarchs, Three Queens, and Two Emperors Decided What Christians Would Believe for the Next 1,500 Years", by Philip Jenkins, and
"A New History of Early Christianity" by Charles Freeman (see Chapter 20: "Victims or volunteers: Christian Martyrs").
Doug.