I enjoy how it sounds like the guy didn't even start out by playing the game, but rather started modding right away. I do get that. The coding itself is fun.
But yeah, the guy could also be making an own game, if it was just about the coding. At some point, you do want your mod to be part of a game worth playing.
Modding an existing game is rarely comparable to the complexity of creating the game from scratch.
Adding assets and changing dialigue in and existing dialoge framework are accessible to a lot of people who are not up to designing physics and hitboxes. AAA titles require vast teams of people to get the skeleton and muscles of the game right, and mods are generally the hair coloring and nail polish.
That isn't to say creating mods is inherently easy or anything, but it is tweaking an existing thing and not comparable to building it from scratch.
Sure, but some game engines out there do give you quite a bit to work with, too.
And well, I was talking specifically about the guy enjoying the coding without necessarily caring about the end result (like he'd do, if he patched up Starfield without enjoying the game).
If that were the case, even designing the physics can be a fun riddle.