Is it okay for the airport bus to operate like this?

I took the airport bus route from Seohyun Station to Gimpo Airport.
It wasn't completely full (though there seemed to be about 10 seats left even if it were), but there were many passengers on board.
As those who have taken it know, it goes straight along Olympic Expressway and then exits toward Gimpo Airport before reaching Gimpo.
Today, however, it veered off next to an old hotel near Yeomchang Station, drove along the bus lane, and then went to the Domestic Terminal first instead of the International Terminal.
There was a flight attendant on board, and from what I overheard in her conversation about a late flight, it seemed to be the case.
I wondered if the bus driver was allowed to do this.
Isn't this too self-serving? There were over 20 passengers on board,
including several foreigners who might have gotten off thinking it was the International Terminal...
It was a very baffling experience.