The question is very specific to TMP117, it's exposed pad should not be soldered to rigid PCB because the mechanical stress causes additional temperature errors. I don't see an explicite recommendation that the exposed pad should be "in contact" with the PCB. I believe that there will be always a certain gap defined by the amount of solder paste applied to the other pads which can't be reduced to zero, even by pressing the IC down. If you want to further reduce the thermal resistance, you can try to apply a small amount of thermal paste.