Anonymity and/or pseudonym are appropriate in certain situations. In this situation, you have a source you know which is credible (you know their name), and they ask you not to publish their name. In such a case, it is standard journalist ethics in The Netherlands to not disclose the name. Sex crimes and exploitation are two (good) examples of such, but there are other examples as well. Consider for example a whistle blower, a (former) member of a cult, or -to put it generally- someone who can, realistically, be threatened when their real name is released. Such is the very case here. I am in awe that The New York Times does not adhere to the very same principles as the Dutch media do, although I am aware that the vow has been broken here as well (such as in the case of Rob Oudkerk and Parool's Heleen van Royen).

It's also appropriate in cases where the subject's real name isn't relevant to the story in any way. That's certainly true here.