In the early days it was a bit more than that. You had to deal with fiddly software and do things like mapping buttons to video tracks, create transition videos, create chapters etc.. It's been a couple of decades since I last dabbled with this stuff, but I remember that doing it well, and creating something that stood out and worked well, was a bit of an art.

Edit: All/most of those things became easy(ier) as authoring software improved, of course, but in late 90s and early 00s it took practice and skill to achieve a professional result.

It's pretty much covered by the other person's answer, but there was a really tight spec to meet "proper DVD standards", and loads of knowledge and stupid workarounds specific to the DVD Authoring software you were using, or CRT-era television standards, or to know the disc would play back properly on different brands of DVD players, but also work on computers (and extra messing around and lower bitrates for it to play back correctly on Mac computers).

Basically a tonne of weirdly specific technical knowledge that's just no use any more.

These days you can sort of just record a video on your telephone and play it back on a telly or the internet, but you used to have to "cap white level at 235" and "lower max red to 240", and export your audio file 1 frame shorter than the video file it accompanies and set fading in titles as an "in-between" video, which links to a looping video with the "real" menu over it and other stuff like that.