I recall reading this quote a number of years ago, and I even recall the uneasy feeling that it gave me when I first read it. In the first sentence he asserts in essence that the measurable and audible quality differences between monopolar and dipolar woofers are due expressly to differences in their respective interactions with the room. This statement is not sincere if it happens that the primary reason for the differences between the two types of woofer is the mutual cancellation between the front and rear waves with dipole woofers. In order for him to have been fully honest and open, he would need to have written something like, "The investigation showed that the primary source of differences in the quality of monopolar and dipolar woofers is not cancellation between the two out-of-phase wavefronts in a dipolar woofer. Rather, the investigation showed that the primary source of differences was the way the two kinds of woofer interact with the room." The thing about the elephant in the room is that whenever anyone who isn't blind seems not to see it, they are only pretending not to see it. Dipole and monopole woofers do differ in the way they interact with the room, but this is mainly because with dipole woofers at very low frequency, there is barely any interaction with the the room, because of the mutual cancellation.
For anyone with a genuine desire to have a more informed understanding of this question, there are simple experiments that may be done. If you have a subwoofer with two active drivers (and no port or passive radiator), you can simply reverse the polarity for one of the two drivers. If you think there might be a difference for the case where the two drivers are facing the same direction vs. facing the opposite direction, then you would need a subwoofer where the two drivers are facing opposite directions. Or you can use two identical subwoofers, and for one of them, you can reverse the polarity in the signal feed. You will likely find that the effect is not the least bit subtle, no matter how you orient the two subwoofers relative to each other, except when your listening position is much nearer to one of them than to the other one. A commonly used way to tell whether two full-range speakers are wired with the same polarity is to stand equidistant from them and listen to how much bass there is, and to listen to whether the bass increases or decreases when you move from a location that is much nearer to one than the other, to a location in the middle. Another revealing experiment, which everyone who has ever tinkered with speaker building has done casually and on multiple occasions, is to simply listen to the sound of a woofer that has been suspended in free air. If you stand a few feet away from it and listen, it will sound very much like a midrange driver and not at all like a woofer. The primary and obvious reason for the difference in the sound quality between dipolar woofers and monopolar woofers is not the difference in the way they each interact with the room. The primary and obvious reason for the difference in the sound quality between dipolar woofers and monopolar woofers is the mutual cancellation between the front and rear waves with dipole woofers.