Soldering temperatures are normally in excess of 200°C (400°F).
The soldering guidelines for Xilinx Pb-free packages are here. Usually lead-free soldering requires somewhat higher temperatures than eutectic Sn63-Pb37 solder.
It's not a problem to have brief exposure to soldering temperature of 220°C or even higher.
The max operating junction temperature is the maximum temperature of the IC die with voltage applied (typically 125°C for an FPGA). High current densities on a chip combined with high temperatures eventually can cause failure due to metal migration.
The outside would have to be much cooler than that. The maximum storage temperature is the long-term maximum temperature with no voltage applied (typically 150°C for an FPGA). High continuous temperatures can degrade the package or cause impurities to diffuse in the chip.
Soldering is a short-term one-time (or occasional, at most) exposure (measured in seconds), so the damage caused, for example, by diffusion of impurities, is limited.