I suspect the drinking occured AFTER Check in, where they will have had a couple of hours to put it away without any restrictions.
This night flght to IBZ (c22.30 t/o?) was perfect for these 'pre-loaders' who clearly had a skin full before boarding. And nobody noticed?
Any takers on the bet that these were off on a 'stag weekend'? Its called "not sh---ing on your own doorstep".