The OPs post seems very clear to me that they are saying the flight wasn't displaying on the board at all, until very late at which point it appeared but didn't show a gate.

I'm also not sure how they were supposed to have made sure they were at the gate 21 minutes before departure if they weren't given a gate number to go to? And other people in the lounge had the same problem. Maybe I'm being dim but they seem to have been expected to somehow guess the gate? Yes I understand it would have been showing on screens outside the lounge, but who on earth would think the screens outside would show anything different to those in the lounge? Having asked the airline staff in the lounge as well I think most people would assume there's not a lot more they can do, are people suggesting they go and queue up at the general customer assistance desk in the terminal with the expectation they will have better information? I don't think I´ve ever seen it suggested before people should be suspicious of information given by lounge staff.