Great post! I always thought that the keyboard scaling was entirely for musical purposes. Such as emulating the difference in timbre in a plucked instrument as you go up in pitch.
One of the main DX7 patents talks about adding an averaging filter in the path of the feedback mechanism. Averaging the current sample with the last one helps prevent the high-frequency aliasing. There's also a 16Khz low-pass filter on the main output.
Yes, you're right. I could have made that clearer. I think the filter is just there to remove harsh high frequency artefacts from the final output for the sake of the overall sound.
One of the main DX7 patents talks about adding an averaging filter in the path of the feedback mechanism. Averaging the current sample with the last one helps prevent the high-frequency aliasing. There's also a 16Khz low-pass filter on the main output.
Here's a very in-depth look at the behaviour of feedback FM: https://ristoid.net/modular/fm_variants.html