People did this with newspapers back in the day. With headphones before Bluetooth took over. With books.
It's not a new phenomenon. Not everyone needs to have small social interactions every moment of the day or even most moments.
We live in a way more connected world than we ever have before and people do not take enough time to themselves because they are always reachable always "on". So especially when you live in an urban environment and take public transit or similar that time you spend there listening to music or a podcast or just shutting out the world can also be healthy.
I question how they controlled for this and if the headphones use is the actual problem or if they are isolating for other reasons and the headphones are a symptom.