Coin cell battery retainer, problems and best practices
Yes, the entire exposed area is electrically connected. The intent is to place a small bump of solder inside the square to improve the connection to the battery.

The reason for the rectangular slots is to serve as thermal reliefs, similar to what is done for plated-through-holes. Without the slots, you would have to heat up the entire circle of copper before any solder would melt. And even then, the solder could flow over the entire area.

The narrow "thermals" made by the slots limit the transfer of heat. Now you can get the square hot enough to melt solder without warming up the whole board.