Transfer function of a second order capacitive accelerometer
your approximation of the sensing element might work (although there is not enough data in the datasheet to back it up), but your are missing the filtering that is being done (the cutoff frequency you mention is for the filter, not for the sensing element). The capacitive filter filters the output of the sensing element and removes those high frequency components, but since the resonant frequency might have a really high peak (the datasheet doesn't mention the peak value) it might still show up fairly well in the output of the whole accelerometer. 

Here is a quick sketch of what I mean by that,