Growing up, I had always assumed my cats didn't care didn't when I left them alone. And maybe those cats didn't. But of the 3 cats I've had as an adult, I've been able to get recordings of 2 of them standing at the door and meowing for a few minutes when I leave the house. They also run out and greet me and my husband when we return home. Those two never even met each other, so they each developed those behavior independently. They each had another cat at home to keep them company - but they really do have a strong social attachment to their humans. The other cat... not so much. But I definitely am not surprised to see that studies are finally confirming that some cats are socially quite clever and capable - but it's highly dependent on the particular cat, relationship with the human, and the environment they're in.