Why can't the author do the same thing Firefox does, which also uses Widevine DRM with no issues?

RTFA. Widevine is owned by Google. Google gives Firefox permission to use Widevine. Google took 4 months to give this individual a one-line rejection, ostensibly his only option for supporting DRM in a chromium-based solution like the one he is using (Electron). Now what?

I did RTFA did you? TFA has no mention of Firefox or Mozilla, nor why the download at runtime approach that Firefox uses is not viable for them. Firefox doesn't bundle widevine in their distribution. That changes things here. Maybe it's still not viable for some reason, but TFA certainly doesn't go into it at all.

Everyone has the ability to play non-obscure videos i.e. all of YouTube. (Including Premium originals!)

How did "DRM video player" become part of the "reasonable feature set"?? Don't most people use dedicated apps for Netflix anyway?

If the user can't play videos from Netflix, then they are left with an uncomfortable subset of 'FANG'.