This is a quirk of the relevant table looking for a use, and not finding one because the actual action using him (hiring his services) is invisible without a money purchase (i.e., playing 12 paid stories to buy Johnny) so can’t be listed on the wiki.
But, at some point Fallen London added the requirement to the action everyone can see, and no one noticed until today! So that’s been fixed and the table now reflects that Johnny Croak “has a use”. (I was prepared to force that action to “fake” using Johnny Croak to fix the automatic searches, but it’s much better that we don’t have to!)