I live in Latin America and I highly doubt that the "missionaries" in that book did any converting at all. They boast about not "proselytizing" or "shoving their faith down anybody's throat."
I've seen dozens of these fresh-faced liberal youngsters, with a few older women in pantsuits and avuncular, live-and-let-live type men to accompany them, come down on "mission" trips and they don't behave any differently than any typical foreign aid agency, whether it be the Peace Corps, OXFAM, the Qatar Foundation or whatever. They purposely downplay even the little bit of watered-down Catholicism they may or may not have, promote indifferentism, laxism, ecuмenism, and liberal college-kid ideas in general. The adults who come with them are all products of the Vatican II revolution. The villagers typically have recieved no proper catechism at all and their religion is a mix of folk beliefs, Vatican II doublespeak and liberation theology.
By far, though, the worst aid agency of all is Caritas. It has probably done more to drive millions of Latin Americans into the evangelical sects than the last five popes combined!