Alex, the point wasn't that the impedance would be a constant 8 ohms. A previous post appeared to suggest that a 4 ohm speaker was the result of two 8 ohm drivers connected in parallel, and that separating the wires in the manner described would increase the speaker impedance substantially above 4 ohms. It's certainly recognized that impedance isn't a constant and varies widely with frequency.
The purpose of the simplified calculations was to show the effect of the crossover circuitry on the impedance. The formula used is the basic one for calculating impedances in parallel. An 8 ohm impedance(without the crossover effect)for both drivers was selected for convenience and familiarity, not because it was a fixed value. The well-known effect of the crossover circuitry to increase the "seen" impedance of the driver not playing the particular frequency by a very large amount results in the speaker impedance(at that particular test frequency)being just slightly lower, not cut in half(two 8 ohm drivers forming a 4 ohm speaker).