Its was not level cruzing at 560 knots. Its had decended from a much higher altitude. Published cruzing speed is 459 knots so 560 is not out of wack. SO while this speed may push the design limits, it does not mean it will break apart. Manf. always pad the numbers for safty reasons and long term durability. Less stress on the air frame, the longer the life as well.
So I disgard this alleged speed and durability aspect of the conspiracy.