The 7-8 people could have been on a SQ flight or another airline departing from that part of HKG (ET & CA also use gates near the SQ lounge).
The vast majority of *A flights depart from another part of the airport and that's where the TG and UA lounges are.
It's hard to say from the limited detail in your story but it sounds like your parents are flying an airline where it makes more sense to use the TG or UA lounge and if the SQ lounge was near capacity then the attendant felt it more prudent to send them there and reserve remaining space for passengers departing from gates in that part of the terminal.
I do agree, if I have a lot of extra time on my hands, I'd rather use the SQ lounge (other than the fact it has no restroom) because I like their food more than the TG or UA lounge, but if I were on a connecting flight and not coming from the immigration area then I wouldn't go out of my way to get to the SQ lounge.