The Malaysians are saying it's the F/O on the radio because the captain flies the plane, the co-pilot does the radio. There's no PF or PM to those unfamiliar with how sectors are operated, and those giving the media briefings don't actually know that first officer also often gets to fly a sector in the modern world.
That's my understanding, from the Malaysian press briefings. I wouldn't place any value on what the Malaysians are saying as to who made the final radio call, I'd wait until someone knowledgable like the NTSB comes forward with their information.
Or explains to the Malaysians how a commercial airliner works and how the crew retain currency in their operation.