Let's not confuse the two types of "frequency" discussed here:
Frequency in sampling theory denotes a simple periodic wave in a Fourier series to rebuild a complex, non-periodic wave in the spatial domain. If you convert a (spatial) scene to the frequency domain, ANY object has an infinitely high frequency component for its edge.
In a hypothetical ideal camera system, any single edge or line will exhibit aliasing artifacts. In practice all camera systems are limited regarding sharpness thereby blurring edges (by diffraction for example). At high enough resolution, the lens will act as an automatic anti-aliasing filter, which is why high resolution systems are less likely to exhibit problems.
Given the above, if a system creates aliasing problems in an edge, then a repeat of edges will result in a fairly large area of inconsistent colors usually visible as irregular patterns known as moire. This repetition of edges is also referred to as frequency, but can be very much below the sampling resolution of the CCD and still result in the moire artifacts...!
Following are very small crops from 5d images from the dpreview site. (I did not ask permission to copy these so I would like you to see them as "quotes" which I presume is allowed under normal copyright laws.)
Images 1 shows aliasing artifacts, image 2 and 3 show how a single image-line (larger than a single scan-line) can already produce discoloration. Given enough single lines relatively close together (but obviously well below sample resolution), one may see moire...