Dan,
Sounds like you are only hearing one part of that communication. What more than likely happened was that JetBlue 178, who was monitoring the guard fequency, inadvertently switched over and transmitted on 121.5. He was out of range for your receiver to pick it up. The pilot that you heard was able to receive JBU's transmission and was kindly informing him that he was transmitting on the emergency frequency.
This happens more now since 9/11. Fortunately there are plenty of "Guard Police" out there. You may also here the once funny response "touche," to an "on guard" transmission.
w0x0f