Singular they was used by respected authors even as far back as the 19th century.
indoordin0saur is correct. Traditional use of singular "they" was restricted to persons of unknown sex, where it is correct and unobjectionable. But the article uses it for persons of known sex. This is a modern innovation, and it should be resisted because it reduces the clarity of the writing.
They were people of unknown sex. Keeping the gender unspecified is part of the anonymity.
Requiring to identify someone's gender when that person is anonymous is just pointless bigotry.
You're already making up fictitious names, so how is making up fictitious sexes any different? By using non-unisex names you're implying specific sexes already. It's implausible that you would know somebody's name and details about their working conditions without knowing their sex.
Alternative solution: abbreviate all the fictitious names to single letters. This is commonly understood to mean obviously and intentionally concealed identity (e.g. "M" and "Q" from the James Bond franchise), which returns the singular "they" to traditional and unobjectionable usage.