Who's Been Impersonating This ProPublica Reporter?

If the impersonator was asking about Ukraine, rest assured that was a Russian spy. Ukraine is kicking Russia's ass pretty well right now with UAVs, so that's why they are trying to get information on them.

Judging by the relative scarcity of instances like this being reported, I would guess that they are successful enough to be a ongoing source of intelligence.

Apart from the subject matter (which also points to Russia) if it were not Russia doing this successfully, they would be motivated to do this at a much larger and more obvious scale.

Fake news is obvious and pervasive, because it is trying to be obvious and pervasive. The goal is not to make people believe the falsehoods (although they occasionally have luck there) but to make people doubt the truth.

If impersonating reporters was not working for them for intelligence gathering, or they knew someone else was doing it. I think they would apply some of their misinformation resources to do massive wide scale obviously bad impersonation of reporters. It would create an atmosphere of suspicion that would dry up sources everywhere.