The issue i suggest is this. The Captain is niether trained nor equipped to judge the serousness of a bomb threat.
He is trained to get the aircraft on the ground Asap after a bomb threat.
He is not trained to deal with a bomb threat on the ground.
Last time I looked, there is nothing anywhere I know of that advises about dealing with a bomb threat by confining people in the vicinity of the threat.
End of story, Unless anyone can provide a logical reason about minimising harm through confining people on board.
Failing that, the pax have the perfect right to take matters into their own hands and deplane any way they can.
My guess would be that the Captain of this aircraft was mindful of the costs associated with an emergency evacuation of the aircraft and decided to take the "low cost" option.
P.S. Thats why I sit by the emergency exit if possible. I'd rather be on the outside helping others leave than on the inside trying to get out behind half a dozen obese clowns.