Assuming they mean the ground acts as a heat sink, and sufficiently underground you’re not subjected to the above average heat of the day and below average cold of the night.
It is not (it has a 88d year, and a 58.65d.. day[0]) , we just had a post about it - if you travel at 4kph you can chase the sun.. A Mercury Rover Could Explore the Planet by Sticking to the Terminator (18 points, 1 week ago, 6 comments) https://news.ycombinator.com/item?id=47720941