I have a hunch that patent writers tend to omit certain details about an invention, so as not to 'give away the store'.
The article tells how sines and cosines are derived "by means of an approximation which forms the "heart' of the converter." As far as I can tell it's 'almost' but not totally explained in the diagrams and text.
There is a network of weighted resistors but without values labelled.
It's possible the author discovered a mathematical relationship between sines and radians, resulting in a particular waveform which he could apply to achieve more accurate results. Etc.
And perhaps since then the inventor (or another person) found another method to do the same task, or a simpler method. Etc.