Usually on a 2nd-order crossover (12dB/octave) one of the speakers will be wired with reverse polarity (either tweeter or the woofer, but not both).

Maybe this documentation is a hint about when you use (or do not use) the crossover on the amplifier, you should not (or should) wire one speaker in reverse polarity?

I wonder if the crossover on some modern amplifiers, when switched to "on" with a slider or switch on the amplifier, will also reverse the polarity of one of the channels for you? Seems like an interesting idea. I wonder if it would add more confusion or less.