They're set very tight.
IME - they respond best to a sudden sharp torque rather than steady pressure. If you have a friend with an impact wrench, stand the hum on a 5mm hex hey in a vise, and use the impact wrench to pop the end cap loose.
I don't use an impact wrench, but sort of duplicate the effect by standing the hex hey in a vise, and using a long arm key with a sudden sharp effort to snap the apart.
I've had the same issues taking apart cranksets. It seems that Campy uses a vanishing oil that dries to where it has a slightly adhesive effect when they assemble components. It takes that sudden burst to pop parts free, and I often detect a "gunpowder" odor when I do so.
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