I'm sure not everyone knows this, because there are are people on this site who have not grown up with phone books.
The article talks about a reverse phone book: according to the author, given a phone number, you are able to look up name (and profile picture). To my understanding there were no reverse-lookup phone books like that back in the day.
-In the good old days (sixties, presumably also earlier) police forces in at least some countries had phone books sorted both by address and by number.
(Source - flea market find, a Norwegian book on the state of the art in criminology, published c. 1965)
Edit: Oh, I just remembered that locally, the athletic union published a phone book with all subscribers in the municipality listed by number.
This was c.1990 - after call ID was an option if you bought a decoder (or was an ISDN subscriber), before Internet phone directories were a thing.
The article talks about a reverse phone book: according to the author, given a phone number, you are able to look up name (and profile picture). To my understanding there were no reverse-lookup phone books like that back in the day.