It is a broad statement, and keeping them as a pair is usually terrible. When you've done what I've done and seen what I've seen, you will see it that way too.
In addition to what Kasia mentioned…
Glad you brought this up as it illustrates a common logical error. Humans, and many other species, are social animals. They live and thrive in groups and frequently in pairs. Red tail hawks pair up and they literally rule their roost as a team, for example. Tortoises, on the other hand, are not social animals. They don't like, want or need other tortoises around. Other tortoises are seen as territorial intruders, bullies, and competitors for limited resources. Tortoises do best when there are not other tortoises around, and they actively drive the interlopers away, one way or another. Assigning human attributes or emotions to tortoises or other animals is called anthropomorphism, and doing so can cause all sorts of problems.
