I'm not sure that's what it's saying, I think the headphone is an FM radio (cordless). I had to read it a few times but I'm still not positive.
I think in this case they were talking about a headphone radio so small that it is barely bigger than your usual headphones. But Aiwa did a an FM transmission unit to make wireless listening possible. It was a separate add-on, though to make the walkman (HS-F2) compatible with their radio headphones (HR-S01). Marantz/Unix also have FM transmitting walkmans, but I don't think theirs was designed with broadcasting directly to your headphones in mind. They're compatible though, since it's basically just FM radio. I tested my Marantz CP-53 with my Aiwa HR-S01 and the transmission quality was very good. I can still get good signal across the room.
I've picked up at least one other player with FM Transmission ability, I think your right, they just go to an FM radio.