How does this circuit output 25kHz?
This can be simulated online using Falstad’s circuit simulator.  If you know the primary inductance, turns ratio and Mutual Coupling ratio (&lt;1) then the impedance ratios are computed for each resulting inductance and referred nonlinear load impedance.. Although not easy, you must know all the RLC values of each trace, capacitor and choke, as well create a model of the trifilar center tapped transformer using two transformers with some awareness on transformer theory.
Transistors are simple with only hFE, FETS are simple with only Vt, gm or inverse RdsOn for the models. Stray inductance and capacitance as well as DCR and ESR must be estimated and added. Diodes are based on physics with Vf@ 1A may not be a perfect model unless you add Rs bulk resistance based on inverse Pd rating or datasheet curvesfor saturated series R slopes.
 Caps are always ideal so ESR MUST be added.
Same for chokes with DCR. and even better with PCB trace inductance if significant.

The series cap and diode clamps the AC to -Vf thus resulting in a AC voltage ratio and positive feedback loop to satisfy the Barkhausen Criteria for oscillation based on your formula above for effective L and effective C which is connected a certain percentage of the time  when the base emitter junction is conducting in series..  

There is also an LC resonance with the AC bridge side load as well as the ferrite core stray capacitance and its own self resonant frequency chosen with winding patterns to be sufficiently high.