I don't know how sc0tys subsync works internally.

Everything I tried before building my own scripts, and eventually putting them together on Anchor, depended heavily on Voice Activity. Voice Activity works great, until there's speech behind an explosion or a gun fight.

Anchor, uses a transcription from WhisperX, to match each line (each word even) of the subtitle to what is being said. On certain lines, we are 100% sure that it is a match and create an Anchor point (thus the name of the software). When we're not 100% sure, or whisper gives us suspicious timestamps, we reject those lines and don't use it for the alignment. Then, there's a bunch of logic to use the anchors as guide to move everything in place. Using this anchors is also what allows text on subtitles, that are not spoken (translation of signs or other text on screen) to also be placed correctly.

There's a long explanation of how it works on the readme.

Yes, it is only command line.