Exactly. If you look at the freq response curves for the mics that are generally used, they start to fall off around 20kHz, so the freq spectrum above that won't be accurately represented; however, as Matt says, the mannor in which the analog wave is reproduced digitally through DSD and WSD/DXD is more true to the analog than PCM (esp true at 1 bit 5.6MHz) making the recorded sound more natural with improved imaging.
From my limited understanding though, DSD has to employ a HP filter to eliminate some of the LF noise generated through the digital conversion.