They specified "good" heavy armor. Ring mail is, unless you have negative dex,
worse than most medium armors.
Curiously, if ring mail is so bad (and I agree that it is bad)... why have it in the game?
I don't just mean why have it in BG3. I mean why have it in DnD?
Seems like there should be some options for a character when it comes to armor, but as far as I can tell there's always one best type for each category. As a player of BG3, I find it frustrating--even if only aesthetically--that all of my armor wearing characters pretty much have to wear the same stuff through every playthrough.
A high dex character can get away with light armor. Otherwise, it's scale mail until the gith armor becomes available.