Yes, that is true, but typical car subwoofer boxes usually have volumes much smaller than optimum. If those super-woofers are working in the proper volume, air nonlinearities are not an issue. I have build several subwoofers with those super-woofers (Audiobahn 12" used to come in a wooden crate and weights a ton!) - all of them have highish Qts parameter and require big box volume.