It’s alway been hotter in cities. Read about the summer of 1776 in Philadelphia, nearly 250 years ago. That’s before pavement. Anytime you have anything that absorbs heat, buildings for example, that heat is retained and takes longer to cool.
It has been true since the first house was built on a rock.
Rocks are know for their stability, to make a great foundation, but rock heats up differently than say, a tree house.
Not sure why these “researchers” felt that this was newsworthy but I suspect that they wanted a little free press, before their next grant submission.