I think we have a different view of spying, this can also be Russians just want a prisoner swap.
All intelligence services have people working for them undercover, you can be hired by or talked into doing x and y by an some intelligence service, you can also be asked to share information with them. They can even help you getting in contact with the right people to make what ever you're trying to do easier.
You can say that a journalist will publish it findings, but also the journalist can do a lot of other things. I mean I can think of multiple things that the CIA would be interested in that is not necessarily related to the journalistic job. Like recruiting a spy inside Wagner, milking information that is a secret: locations, personnel or arsenal. It can also be a damageing story, Russias official stance is that Wagner is not controlled by the state.
Is he a spy? Does he become a spy if he is spying?
Anyways we don't know, Russia maybe just wants s prisoner swap, and maybe this journalist was digging in places that he shouldn't have. We arrest Russian and Chinese spies all the time, some of them are working as journalists.