Sounds a bit ridiculous to me.
To be fair, this is pretty common in the US and isn't a bad practice...the ground power or APU or whatever the source of AC is when the engines arent on is often insufficient to keep the plane cool on hot days. Doesn't surprise me this is coming up now seeing HK has faced record temperatures this summer.
I was on a flight earlier this year where a group of passengers were utterly berating CX flight attendants for the temperature on board. We had boarded but were parked at the gate with a push back delat. 773 to TPE IIRC, I was in economy and the waaaay back. ISM eventually threatened them with escalation to the captain if they didnt shutup. The plane was indeed hot and apparently they had a few elderly passengers in their tour group. It was a situation where they were being over the top and all piling on top of one another to add their piece, but at the same time the plane was damn hot. The passengers couldve behaved better....but also wouldn't have had an issue if the plane was a bit cooler.