Take note of the environment. There's no enclosure, just a big hole they install over the top of, and their suspension is relatively stiff.
With the low end being amplified by the door drivers, I doubt they get loud enough to distort. I know for a fact my aiwa stereo uses a similar trick:
I've taken up collecting those speakers after taking apart the original set in a fit of curiosity. There is a subwoofer with its own enclosure and also a high frequency driver in a separate enclosure, with extremely stiff suspension--the crossovers were designed into the main amplifier stages, so the speakers themselves don't have any extra components, but the suspension and independent enclosures lend themselves perfectly to acoustic filtering.
By wiring the speakers in series, they work with any stereo, and sound quite good without any audible distortion well into the "neighbors asking to turn it down" range.