Optical Spatial Frequency Filtering of Image Sensors ?

Firstly the issue discussed about microlenses isn't about microlenses at all, it is just about having a non-zero sampling aperture. As the sampling aperture increases towards the sample period, the high frequency response will fall. However, it is not worth getting vexed about, because the consequence of a zero length sampling aperture is zero quantum efficiency. The solution is to increase the sampling frequency so that the loss in the desired passband is negligible.

As to the optical low pass filter not being a low pass filter, this is nonsense. All low pass filters, be they RC, LC or FIR, work by 'spreading' the input signal, just like a birefringent OLPF. The line 'The pair therefore splits a ray into four, designed to hit exactly one of each of the four Bayer color filters. In order to achieve this goal' is just nonsense. The AA filter is not a sampled system, there is no way that each output ray could be guaranteed to hit exactly one of the four Bayer filters. That much is enough to indicate that the author is having difficulty conceptualising what is going on here.