Why do we need pull-up resistors or tri state pins for AVR external interrupts?
Activating the pullup resistor is completely independent of detecting the signal. If the pin will be actively driven at all times then there is no need to activate the pullup, and in fact doing so will increase current consumption when the input is pulled low.

On the other hand, many external interrupt sources are actually open drain outputs, which means that they are not capable of actively driving the input high. In this case we must activate the pullup resistor in order to prevent the input from floating.