It is a joke, Argentinosaurus is not considered to be a bird. The joke kinda comes from the idea of crown- and stem-groups in cladistics.
Basically, the crown-group is a group of organisms that includes the common ancestor of all living members of a clade, and all descendants of that ancestor. So, “crown-birds” is the group that includes the ancestor and everything descended from the ancestor of all living birds.
Stem-groups, on the other hand, is a category that includes all extinct groups that are more closely related to the crown-group than to any other living organism. So, in relation to crown-birds, all dinosaurs and pterosaurs can be referred to as “stem-birds” because they are more closely related to birds than to birds’ closest living relatives, the crocodiles!
Stem-groups and crown-groups have fairly limited usefulness in actual taxonomic discussions, but it is genuinely a scientific term and it’s just fun to refer to all dinosaurs as “birds”. Triceratops is Pointy Bird, Tyrannosaurus is Bitey Bird, and clearly Argentinosaurus is DA BIGGEST BIRD