Being outside and socializing is doing a lot of the heavy lifting here...
He found that expert brains were wired differently in regions important for functions like attention, visual perception and visual recognition.
It doesn't elaborate, but my favorite example of brain plasticity well into later life is London Taxi drivers.
It may have changed, but they need to memorize every fucking street in London and know how to get from any two places to get their taxi license or whatever they call it.
So they did brain scans on people before and after and saw huge increases in the "navigation" region of the brain even for drivers in their 60s.
Nothing specific about birdwatching is "rewiring" it's just compared to not using your brain, using it improves it.
It's good to get out in nature and get the physical exercise, but there's a limit to how much of even that an 80 year old can do. Plop em in front of Forza tho and they can game 4-6 hours a day easy, getting expontial gains compared to a few hours a week birdwatching.
But you want as much variation as possible, a smorgasbord of activities is better than any single one.
There's more options than the classic "get hyper specific identifying variations of one thing". But that doesn't mean people shouldn't do that if they're into it either.