Turns out that the key words above are "before buttoning-up." In a proper installation the board with big FPGA is installed such that you put a silpad between the chip and the big cold finger the chip goes up against when the board is installed. if the silpad isn't there and the board isn't screwed into place, the FPGA gets warm. Warm enough to no longer meet timing.
Sure, the temperature specs on FPGAs are junction temperature. If you are running commercial temp range, if the junction gets out of that range the timing is not guaranteed. No different from running your car with no coolant. Not the car's fault and not the FPGA or tools fault.
Right .. remember the tools tell you whether the design
will work over the stated temperature/voltage range.
They don't tell you whether your design will or will not work outside of the range. See, it might, or it might not ... so, you must assume that it won't.
And we proved that, hey, yeah, when it's outside of the temperature spec, it doesn't work.
And further is that you can never tell exactly what
won't work outside of spec range. Do you feel lucky?
This is why I never overclock a CPU.