PFC - Output capacitor Formula
the average current through the capacitor must be close to 0 more than
  Iout


That is entirely true and why should it be a surprise?

We know this: -

$$I = C\dfrac{dV}{dt}$$

And we know that "v" rises (during charge) and falls (during discharge) but, on average remains at a constant value therefore, the average current into the hold-up capacitor is zero.


  Ic would be equal to Iout ?


Not true except when C is providing the current during the hold-up period but, after that period ends, the capacitor is recharged and thus, the average current becomes zero again. Current flows out of the capacitor and current flows back in.