I dont know the full story but i wouldnt accuse the ground staff of "lying." Ground staff arent exactly watching the check in numbers tick up. They are aware at the first start how full the cabins are. Thats probably why they believe the cabin is full. They checked it again when you said something and did notice that the cabin isnt full.

I would also like to say that the job is pretty chaotic and lots of unknown variables is happening. They probably dont know the full story and just used information based on what they know. Def some misunderstanding going on.