There is some amount of axle flex in all of our cars. Flex is not a bad thing structurally. You certainly don't want a too stiff and brittle axle. Some brake calipers are more susceptible to pad knockback than others. The Camaro/Corvette PBR rear caliper is particularly bad on both stock Camaros and Mustangs that use that caliper in some kits.

I wonder if the axles in the video are original or cheap aftermarket replacements.