Not quite - this is how I see it, although electronics gurus please correct me if I'm wrong!
Think of impedance as resistance. Pickups have a lot of resistance due to all that thin tightly wound wire, whereas the output of a transistor amplifier has much less.
When you combine resistance with capacitance, you make a low pass filter. This is how the basic passive tone control works to roll off top end.
Your guitar cable is like one big capacitor. So when driven by a high impedance source like pickups, there is a loss of high frequencies. When driven by a low impedance source such as a preamp in an active bass, the capacitance stays constant, but as the resistance is less, the frequency cutoff of the filter is moved higher, letting more treble through.