You are not accounting for different types of speaker enclosures and their inherent acoustic roll off, and what happens when they're fed frequencies below that.
Sealed alignments roll off at 12db/octave (typically), but reflex alignments roll off at 24 db/octave, due to the output from the port going out of phase with the active driver causing cancellation. It's still a good idea to cross ported mains well above those frequencies, as it's easier to get a smoother transition to your subs by doing so, so in the case of ported speakers I would agree with the author.
Another reason to keep in mind with reflex enclosures is that at frequencies where the active driver and port are out of phase, the speaker will exhibit excessive cone motion even though it's not increasing in output due to cancellation with the port. This will wildly increase modulation distortion and potentially damage the driver. So, if you take your ported speaker with output to 50 Hz, and send it a signal, high passed at 50 Hz with a 12db/oct slope, the speaker will indeed get significant signal in the lower frequencies, resulting in the problems described.