Well, look at the phrase just a few sentences above - “Authorized to work in the US” – that’s the one which pertained to you at the time.
Most airlines will combine the two ‘requirements' and list them as “US Citizen
or Authorized to Work in the US” or something similar.
IF your personal experience was that you were not hired simply because you were a green card holder – well, you could sue the crap out of them because according to the law you were authorized to work in the US…
Did they actually tell you that's why you weren't hired?