The data sheet says this:
"The heat of the IC is mainly dissipated through the thermalpad. Maximizing the copper area connected to the thermal padis preferable. In addition, a solid ground plane is helpful for theEMI performance."
It DOES NOT explicitly say whether the "copper area connected to the thermal pad" should be directly via - connected to the board's ground plane, or whether it should be a "floating" copper area on the outside mounting layer. Which is preferable?
The thermal pad should be connected to ground. We have a FAQ about this: https://en-support.renesas.com/knowledgeBase/11972719
Also TB389 "PCB Land Pattern Design and Surface Mount Guidelines for QFN Packages" is a good resource and it also applies to our DFN packaged parts such as the ISL97701.