Although luminance information is not only green, there is a well documented loss of resolution caused by the spatial averaging involved in generating the two missing color values at each pixel. The dominance of the signal from "green" pixels (which in reality measure a wide spectral range across the middle of the visible spectrum) means that at a red or blue pixel, the luminance part of the demosaiced output depends more on the data from nearby green pixels than on the signal from that pixel itself. EDIT: a post by "h" quantifies this: even at a red or blue pixel, green accounts for about 70% of the luminance information, which in turn is what dominates our perception or resolution END EDIT.
Or if you do not trust theory, just look at the resolution measurement for the X3 style pixels of Foveon/Sigma sensors, which roughly match a Bayer CFA with about twice the total pixel count, and so roughly the same count of locations at which green is measured.
On the other hand, those X3 sensors with no AA filters still show some aliasing artifacts, because there is indeed more to aliasing problems than color moiré, even though that is the most visible and famous aliasing problem. There are also some examples of "luminance aliasing" in discussions in this forum.
In defence of Leica, it is probably not in a position to avoid the deep, dark, dirty secret of CCD sensors --- the noise problems in very deep shadows and at high Exposure Index --- because adopting a better sensor technology would have extremely high costs relative to its very low unit sales. In particular, the Kodak sensors for M cameras have a special design with microlenses offset towards the optical axis, needed to deal with the highly off-perpendicular incident light delivered by some short focal length rangefinder lenses that have their rear elements amd exit pupil very close to the focal plane (no SLR lens does this). It could well be that even if a sensor supplier like Sony could do this, none except Kodak and Dalsa have developed suitable microlens technologies, and it is not cost effective to have a CMOS sensor supplier do it. Note that Kodak and Dalsa uses those offset microlenses in some other sensor too, like recent 44x33mm models, so that the cost is shared over more sensors, whereas a CMOS sensor maker would probably have only the Leica M cameras with a use such microlenses.