The term implies something that feels like "dodo" or imbecile.
Corvids and parrots and raptors are remarkably smart, yet not every bird species is. Some are incredibly stupid.
Just think of the "Always Sunny" meme where they call Dee a "bird" all the time. It's meant to mean stupid, amongst other things.
The Always Sunny thing is more about her looks. When they call her dumb, they tend to just call her dumb/stupid/etc.
You will find middle age tales with similar tales.
Raptors are pretty stupid. They don't _need_ to be smart. They're solitary and they are typically apex predators.
I have a great view out of my window, and I have a family of crows living nearby. They cooperated to drive out owls and eagles that were nesting in a nearby park. Right now it's raining with pretty strong gusty winds and the crows are flying outside, doing pretty crazy aerobatics. They are clearly having a lot of fun!
I have never seen pigeons or seagulls do that.
Some birds have impressive specializations, and corvids clearly punch above their brain mass weight due to impressive efficiencies with their brain architecture, but I think the framing of the article is more wrong than right to frame it around speedy information processing of particular problems.