Double Midi knows this stuff backwards and forwards. Electrostatic devices, carry a charge, so it’s very likely that it’s impedance is very different from a piezo.
I rather learned from Francis Deck's publications (schematics and discussion forum) with some more basic understanding of electronics.
But back to the topic...
25 KOhms is more typical for a preamp output impedance than anything electrostatic (like a condenser capsule).
Condenser capsules (without built-in FET as an impedance buffer that many cheap electret capsules have) have a much higher impedance (less electrons to flow) than a typical piezo crystal pickup. Therefor the output impedance is much higher. In schematics I have seen resistors around one GigOhms. Electret condensers are still condensers, but they don't need the high phantom voltage for plate polarization because there was made a permanent (at least for some years) charge to one plate. The output impedance from a electret or normal condenser capsule should be in a similarly high range (GigOhm). Because of the high risk of electric noise, the impedance buffering FET (or valve if it's a valve condenser mic) needs to be as close as possible to the capsule. Therefor many cheap electret capsules (the ones with a PCB at their back) have the FET at its back (on the inside, you cannot see it).
But the basic schematic for an impedance buffer for a piezo and a condenser capsule is the same, only the input impedance differs a lot, the voltages of the condenser are a lot lower than from the piezo (as far as I know) and the balancing resistors around the FET (might need to be different even for two pieces of FETs with the same type number) and supply voltage might differ.
But values are always in a MegOhm to GigOhm range, never in KOhms range without impedance buffering (which is kind of a current amplification, not voltage amplification, so the acceptable resistive load is increased but not the amplitude like in a voltage amplification circuit).
So the 25 KOhms are the output impedance of either an condenser with an impedance buffer (most likely an electret that typically has a 2 KOhms output impedance) or the output impedance of the preamp that comes with the capsule. A supply voltage of 1.5 to 10 volts almost always is the supply for the FET of an electret condenser (unless there is a circuit inside that raises the input voltage to 48 volts phantom power).
So I'm rather sure my AKG C411 contact mic is an electret (but probably a better one) with a built-in FET (because the cable to the B23 preamp is much too long for a GigOhms impedance connection).
And I'm sure the Ischell is an electret too. Both are or could be driven with 9 volts and won't work without a supply voltage.
BTW the Shadow NFX piezo foil pickups have the FET directly near the foil under the black plastic, so the high impedance connection is very short. Very similar to the wiring of an electret capsule with a built-in FET. (Piezo foils may need an input impedance to up to 22 MegOhms.)
BTW, Francis and drurb (which seemed to have vanished from here, at least in the groups I read regularly) are the electronics experts. I just try to learn and understand and try to explain it if I feel the need to do so.
