If copper plating cannot plug the vias, the thermal vias can be tented with the solder mask on the PCB top layer. A void in the solder paste will have a negative affect on heat dissipation. There are three ways to dissipate heat conduction, convection and radiation. With heatsinks, large leadframes and heavy gauge wirebond, example TO-92, most heat is transferred off of the die through conduction. In tiny packages such as the SC70-5 most heat transfer is through natural convection off of the die as the leadframe and wirebond is too thin to handle much heat transfer through conduction.
Most portable applications are closed environments without fans, liquid cooled air, etc.
The active side of the micro SMD is bonded down to the PC board and the heat is transferred conductively from the die through the bumps to the PC board. Conduction is much more efficient than convection in transferring heat. An MSL of 1 makes both of these packages hermetic. That means they do not absorb any moisture and therefore do not need bake, dry pack or nitrogen storage. This greatly simplifies handling and board manufacturing.