I've never been hailed, nor heard USCG hail anyone for a boarding. They pull up close and turn on the flashing lights and siren/horn. I've had them come up broadside and ask if I minded being boarded. When I said "No I don't mind", they told me to put the boat in neutral and leave it running (calm day). They were on the boat in seconds after that.
I asked them what they would have done if I 'declined' being boarded, to which they all laughed. The officer in charge replied (smiling) "We would have boarded anyway, but this encounter would not have been nearly so cordial".
I personally could care less if they board unannounced while I'm underway. They're the ones taking the risk. I honestly question the story where the USCG had guns drawn. They chose to jump from one moving vessel to another moving vessel with a sidearm drawn. I call BS on that. I do believe that someone on the USCG vessel may have had a weapon drawn, covering his colleagues, during the operation. I have never seen, and doubt that they would draw weapons once on board without very good reason.
Like most things, these stories get turned into Paul Bunyan tales with each telling and each new person that tells it. Like this one, which started with......"I heard someone say once....."