为何用that is？

有一读者问： 新中学英语教材中有这样一个句子：

It is thought that one billion people, that is half the world's workers, earn their living by farming.

请问为何用that is而不用that（或who）are?

这里之所以用that is是因为that is=that is to say（解释性状语），后接一同位语。