Pani, that page does little to convince me of such a fundamental rule. There can be no impact on time alignment, even with cables of slightly differing lengths -- electronic signals are orders of magnitude faster than sound waves. My Tannoys are not actually time aligned, anyways.
Barring electrical mismatch, I'm skeptical there can be much impact on coherence -- if there was, then I'd have gone back to a single run. It's quite coherent sounding, as to be expected of a Tannoy. Next, look at the crossover adjustments on these Tannoys: you can modify 1.1kHz tweeter shelving, which I've had sound excellent in either 0.0 or +1.5dB positions. That's MUCH larger than the differences between any 2 reasonable cables. Yet the tweeter and woofer can still blend seamlessly in either configuration, as long as it's the right choice for the overall system balance. And then the tweeter/woofer drivers themselves couldn't possibly be more different (2" metal dome compression driver; 15" paper woofer), and yet they too blend beautifully. Again, these differences are far greater than those between cables, yet they don't preclude excellent coherence.