Most of the airlines have a policy to board passengers when the slot leads to an 1 hour delay or less, hoping to improve the slot in the coming tens of minutes. If the slot delay exceeds one hour, the crew decides what to do, board 'em or leave 'em at the gate. That's the game everyday.
The crew don't want them on board in a sticky plane on the ground and the handling agent don't want them at the gate tieing gate personell.
I don't see a mishandling in any way. Can't satisfy SLF anyway, they always complain, put 'em on board, they feel like prisoners, keep 'em at the gate, they want to go to the plane (as it would make any more sense).