The tank isn't the main part of the circuit that sets the frequency. The feedback path that determines it is between the gate and drain of other MOSFET. Ideally, you use them to set the frequency then tune the tank to match it.
I'm still of the opinion this a BJT circuit modified for MOSFETs, I suspect if you remove R2,C2,R4 and C4 it will work just as well. The capacitors slow down the rise of gate voltage which means the MOSFETs spend more time in a partially conducting state where they are least efficient in that configuration.
Brian.