It means that the same change in value in the modulating signal has a greater effect at higher frequencies, which is also a good verbal descriptor of the term “exponential”.
The most well-known exponential curve for synthesis is the volt per octave standard for pitch, which mimics the way the human ear hears pitch: in order to be perceived as a change of one octave, the frequency must double, regardless of the frequency range that is being heard. For example a change from 100 to 200Hz (a difference of 100Hz) is perceived as the same amount of change as a change from 1000 to 2000Hz (ten times the difference).
In the case of the QPAS the curve is tailored so that changes in frequency are even apparently “faster” in higher frequency ranges. So if two peaks are far from each other in frequency, a single modulation seems to affect the higher frequency peak more strongly than the lower frequency one. As they approach their highest frequencies this apparent modulation depth closes together somewhat.
The short version of this is simply to say that if these filters tracked 1 volt per octave, it would in fact result in a compromise to the sound of the complex modulation that is possible from simple inputs. The lack of 1v/oct tracking could even be said to be one of the reasons that the QPAS sounds so good.