That's still considering "modding" your game. You're editing internal files to change settings past the allowed limit. If you can't do it in a vanilla in-game instance, it's likely considered a mod. Both these instances, either editing your options.txt or installing UHC Essentials, their Fullbright option is the same functionality, both needing modification to your game files in order to achieve.
Also, sure, everyone can do it, but does everyone know how to do it? Lots of people, including a couple I know irl, don't even know how to access their .minecraft folder without asking me. So those who have the knowledge of how to access the files will be able to use these modifications. As this isn't the entire playerbase of the server, you can just assume "since everyone can do it, it should be allowed." It may be common knowledge to us regular minecraft players, but just not everyone knows it.