I think you have got it, but you are saying it poorly. RMS applies to any waveform, including the pulses one that capacitor filters apply to transformers. RMS valuation of the current refers to the heating effect of the waveform.
I would say that the capacitor filter requires derating the transformer current rating because the narrow current peaks raise the transformer RMS current to DC output current ratio, compared to having the rectifier loaded only with a resistor drawing the same average current (sinusoidal transformer current wave).