The metal top in any Flip-Chip package is thermally connected to the die with a high-resistivity, low thermal conductivity paste.
The heatsink on Xilinx BGAs with heatsinks (this includes all FF and FFG packages and some FG packages) are considered "grounded" when the part is fully assembled. Prior to package assembly, the heatsinks are floating from all ground ball sites. There is no direct connection from balls/ball sites to the heatsink. However, it is incorrect to conclude that the heatsink is floating because when fully assembled (as seen by the user) the chip does make contact through its p-substrate with the heatsink.
Between the heatsink and the bulk substrate there is a small finite electrical resistance owing to the conductive die-attach paste and a thin CuO layer on the heatsink. The polarity is whatever the chip-substrate happens to be. In the case of Xilinx p-substrate chips, this makes the heatsink assume the polarity of the substrate, in this case ground.