Lost count of the times I have queued up outside waiting to board a Easyjet or Ryanair flight and occasionally but much rarer a BA one.

While a noisy APU is not great I am unaware it is at a level that would damage someone's hearing and if someone is sensitive to noise when travelling on very large and noisy machines like airplanes I would suggest earplugs or headphones to mitigate the issue or to not travel as realistically you may always be exposed to being outside a plane on an active airfield!

This really doesn't seem like a major issue or one that we even know the facts of. Could the captain of maybe said sorry for the delayed boarding, yes but equally maybe he wasn't at liberty to give an explanation and thought better say nothing than draw further attention to things and people. But I don't think for a second they just made everyone stand there to have a giggle and for their own entertainment, for all we know the gate agents got ahead of themselves with boarding.. but everyone made it back without it seems any real delay and no one as far as we know we're bumped from the flight to accommodate these mystery people!