NFHS End-of-Period mechanic

I had a partner with an interesting mechanic I had never seen before a couple months ago. Last second shot of the 3Q, player is coming in transition, pulls up for a 3, releases the ball with about .3 remaining on the clock. Clearly got the shot off in time yet when the ball is in flight and the horn sounds I hear a steady, long, whistle. Beeeeeeeeeeeeeeep.

Shot misses and the carom comes to me. I grab the ball and go over to my partners who are huddling. I ask if someone blew the whistle and the Trail who had been responsible for the clock said he had blown the whistle. I asked if he was going to wave off the shot and he said no, the player did release it in time. I then asked why the heck he sounded the whistle while the ball was still in flight?? and he said "the mechanics book says to hit the whistle when the horn sounds to end the quarter". One of the dumbest and least practical things I've ever heard.

I can't even find what he's talking about in either the rule or mechanics book, does anyone know what he was reading/smoking?