The statement about a low proportion of women who used NFP having abortions, referred to women who had access to contraception but chose to use NFP to space births, living in developed countries where abortion is legal. I think the actual statistic was from the US. This seems very likely to me as, I would imagine, the proportion of women using NFP (not just not using articificial contraception) who are also opposed to abortion is very high. Whilst NFP or FAM can be a good option for couples for a variety of reasons, and has a certain following among feminist hippies, it's still pretty much the preserve of devout Roman Catholics and conservative Protestants.