Three members of the ESV Bible translation committee are very vocal against the use of inclusive language for human beings when it means using different forms in English than the original language has. That's why the ESV tends to translate 'adelphoi' as "brothers" rather than as "brothers and sisters" or "dear friends" as some of the inclusive language translations are now doing (cf. the NRSV, NLT, TNIV, and CEV). The inclusive language translations tend to avoid using masculine pronouns when the group they refer to includes women or girls, and thus some of the inclusive language translations will use the singular 'they', which is pretty much standard in contemporary English but is not really new to English in recent years anyway, despite the claims of those who have resisted it. It's recognized in the Oxford English Dictionary and the Cambridge Grammar of the English Language.
So it is indeed a great irony that the ESV itself contains an unambiguous example of the singular 'they'. Peter Kirk discovered it, and Rick Mansfield has some further thoughts on it. I agree with their general assessment that this is a problem for a translation that explicitly states in its translation policy that it does not translate in this sort of way. That suggests that someone in the editing process did not notice that a translator had done this, either out of a rushed job or because the editor in question, like the translator in question, is so familiar with the singular 'they' that they did not notice. So I'm in full agreement that this is in itself evidence against the view that the singular 'they' is bad English. I do, however, have some reservations about how we might frame our criticism of the ESV on this. In particular, I think we need to be careful not to treat the Grudem-Poythress-Ryken view as representative of the ESV translation committee in general.