yes they are
if its just sitting on a pad there is going to be little air gaps, and air is a good insulator thus reducing its effectiveness, since your not hitting the chip very hard it might not matter as much, best way to tell is to put it in a worst case situation and take its temperature (and adjust for die to body temp, its in the datasheet) ideally when the chip hits its a temp where it just wont get any hotter.
you can solder that pad with some paste and hot air, thats how the machines do it
if theres a pad on the other side of the board with via's you can flux up the chip side real good and heat from the oppsite side shoving solder into a via, it will wick a little
or you can drill a small hole where the slug would be and solder it that way
that is if its needed