Convocation CD Bug

No. This modifier does not apply a reduction to the time of the cooldown, it applies an increase to the rate at which you recover the cooldown - hence the description. This is important, as those are very different things; reducing the time of the cooldown by 100% would mean it takes 0 seconds, whereas increasing the rate at which you recover it by 100% means you recover it twice as fast (taking 4 seconds). Since your total increase to the speed is less than 100%, you'll be slower than that, so will take longer than 4 seconds to recover it.

The base cooldown is recovered at a rate of 1/8th per second, and a 70% increase to that gives 0.2125 of the cooldown per second, which works out to taking 4.71 seconds to fully recover.