> For example, 720 is tied to 13.5 Mhz because sampling the active picture area of an analog video scanline at 13.5 MHz generates 1440 samples (double per-Nyquist).

I don't think you need to be doubling here. Sampling at 13.5 MHz generates about 720 samples.

    13.5e6 Hz * 53.33...e-6 seconds = 720 samples

There's some history on the standard here: https://tech.ebu.ch/docs/techreview/trev_304-rec601_wood.pdf

Non-square pixels come from the legacy of anthropomorphic film projection. This was developed from the need to capture wide aspect ratio images on standard 33mm film.

This allows the captured aspect ratio on film to be fixed for various aspect ratios images that are displayed.

https://en.wikipedia.org/wiki/Anamorphic_format